as already mentioned , the present invention has three main aspects , namely ( a ) double medium displays , and processes for driving such displays ; ( b ) differential thickness displays ; and ( c ) mixed color displays . these various aspects of the present invention will be described separately below , although it should be understood that a single display may make use of more than one aspect of the present invention . as already mentioned , in one aspect this invention provides a double medium display in which a single electro - optic layer contains two different types of electro - optic medium having different characteristics . this double medium display displays a fixed image but the appearance of the image changes depending upon the direction of the applied electric field . in a preferred from of this type of display , the image reverses color when the electric field is reversed ; for example , the image may be white - on - black when the field is applied in one direction and black - on - white when the field is applied in the reverse direction . thus , for example , the double medium display could be used to provide an eye - catching display of a corporate logo or trade mark in a pop display , or in a large , billboard - type display in an arena or stadium . this can be effected by making the first and fourth optical characteristics of the display substantially the same and the second and third optical characteristics of the display substantially the same . alternatively , the first , second , third and fourth optical characteristics may all be different , so that as the direction of the electric field across the electro - optic layer is reversed , both the foreground and background colors of the image change ; for example , when the electric field is in one direction , the image could be black on yellow , while when the electric field is reversed the image could be white on blue . such dramatic color changes are very effective in providing an eye - catching display . although as already mentioned other types of electro - optic media may be used , a double medium display can be conveniently constructed using two different types of microcavity electrophoretic media . it is shown in the aforementioned u . s . pat . no . 6 , 822 , 782 that the charge on electrophoretic particles can be altered by providing such particles with a polymeric coating , which may itself contain charged or chargeable groups . modifying the charge on electrophoretic particles in this manner enables one to produce electrophoretic particles with the same optical characteristics but of opposite charges . for example , titania particles can be produced with both positive and negative charges . by encapsulating such oppositely - charged particles separately with a colored suspending fluid , in the manner described in many of the aforementioned e ink and mit patents and applications , one can provide the double medium display ( generally designated 100 ) shown in fig1 a and 1b of the accompanying drawings . the display 100 comprises a substrate 102 bearing a single continuous rear electrode 104 ; this electrode 104 may or may not be light - transmissive . the display 100 further comprises a layer of electrophoretic medium ( generally designated 106 ), a single continuous light - transmissive front electrode 108 and a front substrate 110 , which serves as a protective layer for the display 100 , helping to prevent mechanical damage to the electrophoretic layer 106 . the exposed surface 112 of the front substrate 110 serves as the viewing surface through which an observer views the display 100 . the electrophoretic layer 106 comprises a plurality of capsules , each of which comprises a capsule wall 114 within which is encapsulated a colored suspending fluid 116 , which is typically dyed blue . however , the capsules are of two different types . one type of capsules , illustrated by the three capsules at the right - hand side of fig1 a , contain negatively charged white titania particles 118 a ; the second type of capsules , illustrated by the four capsules at the left - hand side of fig1 b , contain positively charged titania particles 118 b . as indicated by the + and − signs adjacent the electrodes 108 and 104 respectively , fig1 a shows the display when the front electrode 108 is made positive relative to the rear electrode 104 . in this situation , the negatively charged particles 118 a are attracted to the front electrode 108 , so that the first areas of the display ( i . e ., the areas in which the capsules contain the negatively charged particles 118 a ) display the color of the white particles 118 a . on the other hand , the positively charged particles 118 b are attracted to the rear electrode 104 , so that the second areas of the display ( i . e ., the areas in which the capsules contain the positively charged particles 118 b ) display the blue color of the suspending fluid 116 . in contrast , fig1 b shows the display 100 when the front electrode 108 is made negative relative to the rear electrode 104 . in this situation , the negatively charged particles 118 a are attracted to the rear electrode 104 , so that the first areas of the display 100 display the blue color of the suspending fluid 116 . on the other hand , the positively charged particles 118 b are attracted to the front electrode 108 , so that the second areas of the display 100 display the color of the white particles 118 b . thus , by changing the relative polarity of the front and rear electrodes 108 and 104 respectively , the display 100 can change from a white - on - blue to a blue - on - white image . it will be appreciated that the double medium display of the present invention is not confined to the use of a single type of particle in a colored suspending fluid . for example , the double medium display may use dual particle media of both the same polarity and opposite polarity types ; see , for example , the aforementioned 2002 / 0171910 . for example , in the display 100 shown in fig1 a and 1b , the suspending fluid 116 could be uncolored , while the capsules containing the negatively charged particles 118 a could also contain positively charged black particles , while the capsules containing the positively charged particles 118 b could also contain negatively charged black particles , so that the display could change from a white - on - black to a black - on - white image . also , depending upon the colors chosen for the particles and the suspending fluid , the colors displayed in the state corresponding to fig1 a could be completely different from those in the state corresponding to fig1 b ; for example , the display might change from a blue - on - yellow image to a red - on - green image . as already indicated , in another aspect the present invention provides a differential thickness electro - optic display in which the layer of electro - optic medium has at least one first area having a first thickness and at least one second area having a second thickness less than the first thickness , such that , upon application of an electric field to the electro - optic medium by the electrode , the at least one second area will change from its first to its second optical state at a rate different from the at least one first area . as explained above , there are two principal variants of such a differential thickness display , the internal shim and external shim variants , depending upon whether the layer used to control the thickness of the electro - optic medium is or is not adjacent this electro - optic medium layer . in one process for the production of an internal shim differential thickness display , a thin cut - out or shim having the form of the desired image is formed and placed on an electrode ( typically mounted upon a substrate ). the electro - optic medium is then coated ( or , depending upon the type of electro - optic medium used , laminated ) over the electrode and shim , so that the layer of electro - optic medium formed is thinner in the areas covered by the shim than in the remaining areas . the electro - optic medium is then secured to a second electrode ; typically , the medium is laminated to the second electrode using a lamination adhesive . in an internal shim differential thickness display , a wide variety of materials , including printed inks , may be used to form the shim , but in order that the completed display may function in the desired manner , it is necessary to consider carefully the resistivity of the shim material . in one variant of such a display , the shim material has a volume resistivity lower than that of the electro - optic medium itself in order to ensure that the applied electric field across the electro - optic medium is higher in the areas covered by the shim than in the remaining areas . the shim may be formed of a conductive material , for example a metal or a conductive polymer . however , the volume resistivity of many electro - optic media is so high ( for example , of the order of 10 10 ohm cm for many electrophoretic media ) that many polymers and other materials normally considered as insulators have sufficient conductivity to be used in the shims . in a second variant , a high volume resistivity material is used for the shim so that the applied electric field across the electro - optic medium is lower in the areas covered by the shim than in the remaining areas , so that the change of optical state occurs more slowly in the areas covered by the shim . however , this approach is some times undesirable , since it produces a display which changes state more slowly than a display using a more conductive internal shim , whereas it is generally preferred to use a display which can flash rapidly when desired , since rapid flashing tends to be more eye - catching . a very similar process may be used to form an external shim differential thickness display by placing the required shim on a substrate , covering the shim with an electrode ( and , typically , a supporting layer ) and then coating or otherwise placing the electrophoretic medium layer over the electrode . it will be appreciated that , in an external shim differential thickness display , the shim functions only to affect the thickness of one or more areas of the electro - optic layer . depending upon the materials used for the various layers of the display , the change in thickness caused by the external shim may be permanent , and if so the external shim may be removed after it has served its function of causing the desired thickness variation . indeed , in some cases , the “ external shim ” could have the form of a permanent die which is applied to a succession of displays during their production . such an arrangement has the advantage that a variety of dies can be stored and the die in use changed quickly ( in conventional pressing apparatus that will be familiar to those skilled in the art ), thus enabling any of a variety of patterned displays to be produced on an “ as needed ” basis . when an electric field is applied to a differential thickness display of the present invention , the thinner area ( the message ) will typically be addressed more quickly than the rest of the surrounding area due to the thinner layer of electro - optic medium ( although as already noted an internal shim differential thickness display can be arranged to have a thinner area which is addressed more slowly than the rest of the surrounding area ). by controlling the thickness of the shim and the duration of the addressing pulse , one can create a display where the message switches to a large contrast from the background and then stop the pulse leaving the addressed message visible in a stable state . alternatively , the message could be displayed only transiently by allowing a longer duration to the electrical pulse which would allow the background to address as well . in this case , an observer would see the message address very quickly and then disappear into a uniform background . thus , it will be seen that a differential thickness display of the present invention requires no patterning of electrodes at all , and also requires no special printing techniques . the only things that need to be controlled are the thickness of the shim , and the length and voltage of the addressing pulses . an internal shim differential thickness display ( generally designated 200 ) of the present invention is shown in fig2 of the accompanying drawings . the display 200 comprises a substrate 202 bearing a single continuous rear electrode 204 ; this electrode 204 may or may not be light - transmissive . the display 200 further comprises a layer of electrophoretic medium ( generally designated 206 ), a single continuous light - transmissive front electrode 208 and a front substrate 210 , which serves as a protective layer for the display 200 , helping to prevent mechanical damage to the electrophoretic layer 206 . the exposed surface 212 of the front substrate 210 serves as the viewing surface through which an observer views the display 200 . the electrophoretic layer 206 comprises a plurality of capsules , each of which comprises a capsule wall 214 within which is encapsulated a colored suspending fluid 216 , which is typically dyed blue . however , unlike the electrophoretic layer 106 shown in fig1 a and 1b , the capsules in fig2 are of only a single type , containing negatively charged white titania particles 218 . the display 200 further comprises a shim 220 formed from a conductive material , for example aluminum , and in contact with the rear electrode 204 . thus , the electrophoretic layer 206 is thinner in the areas covered by the shim 220 than in the remaining areas of the display , and since the shim is formed from a conductive material , the electric field within the electrophoretic layer 206 will be greater in the areas covered by the shim 220 than in the remaining areas and the movement of the electrophoretic particles 218 and the change in optical state ( brought about in the same manner as already discussed above with reference to fig1 a and 1b ) will also occur more quickly in the areas covered by the shim 220 . fig3 of the accompanying drawings is a schematic cross - section , generally similar to that of fig2 through an external shim differential thickness display ( generally designated 300 ) of the present invention . most parts of the display 300 are essentially identical to the corresponding parts of the display 200 shown in fig2 and are numbered accordingly . however , the internal shim 220 shown in fig2 is replaced by an external shim 320 , which is separated from the electrophoretic layer 206 by a rear electrode 304 and a substrate 302 . as shown in fig3 , the presence of the external shim 320 causes distortion of the rear electrode 304 and the substrate 302 , thereby producing a thin portion of the electrophoretic layer 206 having essentially the same dimensions as that of the external shim 320 ; fig3 assumes , but does not illustrate , a rigid support contacting the lower surfaces ( as illustrated in fig3 ) of the shim 320 and substrate 302 , so that these lower surfaces are held in the same plane . accordingly , the external shim display 300 shown in fig3 will operate in a manner exactly similar to the internal shim display 200 shown in fig2 . however , since the external shim 320 is not disposed between the electrodes 208 and 304 of the display 300 , the electrical resistivity of the shim 320 is irrelevant , and if the deformation of substrate 302 and electrode 304 shown in fig3 and caused by the presence of shim 320 are permanent , the shim 320 can be removed without affecting the functioning of the display . as already mentioned , in its third main aspect , the present invention provides a mixed color display ( the use of the “ mixed color ” for convenience does not of course preclude the possibility the optical characteristics of such a display may vary in some parameter other than color , for example reflectivity ). in such a mixed color display , the electro - optic medium comprises a mixture of at least two types of electro - optic elements , each type having at least two different optical states . the various types of electro - optic elements have differing switching speeds , i . e ., for any given electric field applied to the electro - optic medium , the various types of electro - optic elements will require differing periods to change optical states . the differing switching speeds of the various types of electro - optic elements increase the range of colors which can be displayed by any given pixel of the display ( which must of course contain at least one , and preferably several , electro - optic elements of each type ). essentially , the differing switching speeds allow independent addressing of the differing types of electro - optic elements , even though those elements are present in a random mixture . although in theory a mixed color display of the present invention can provide a full color display , in practice such a mixed color display will most commonly be used to provide a display capable of displaying a limited range of colors , but a range substantially larger than that afforded by a conventional electro - optic medium which exhibits only binary switching between two extreme optical states . any electro - optic medium in which the switching speed can be controlled and which permits more than one type of electro - optic element to exist in close proximity may be used in the mixed color displays of the present invention . thus , for example , the electro - optic medium may be of the rotating bichromal member type , with the individual rotating bichromal members serving as electro - optic elements , or the electro - optic medium may be a microcavity particle - based electrophoretic medium , with multiple types of electrophoretic media segregated into separate microcavities . for convenience , this aspect of this invention will primarily be described below assuming that the electro - optic elements are individual capsules of an encapsulated electrophoretic medium , since the necessary changes needed to use other types of electro - optic medium will readily be apparent to those skilled in the technology of electro - optic displays . to prepare a mixed color opposite charge dual particle encapsulated electrophoretic display , conveniently two batches of capsules are produced , each batch having a different pair of pigments that have both opposite charge and contrasting colors . one batch of capsules , intended to have a high switching speed and hereinafter called “ batch i ” may be produced using a low viscosity solvent ( e . g ., heptane or isopar e ), a small capsule size ( e . g ., 30 to 40 μm ), and / or be formulated in other ways so as to have a relatively fast switching time ( typically , less than about 500 msec ). the second batch (“ batch ii ”) of capsules is formulated to have a switching time substantially greater than that of the first . long switching times can be achieved by any of a number of techniques , including the use of higher viscosity solvents ( e . g ., isopar v ), larger capsules , or a variety of addenda that modify either the charging of the particles or the viscosity of the suspending fluid . the mixed color display is constructed using a mixture of capsules from the two batches . assuming the switching times of the two sets of capsules are sufficiently different , the display can be driven with a short drive pulse so as to fully switch the batch i capsules but have a very small effect on the batch ii capsules . a much longer drive pulse will switch both sets of capsules . as already mentioned , in a conventional electro - optic display , each pixel has two extreme optical states . in contrast , in the mixed color displays of the present invention , each pixel may be regarded as having four extreme optical states . for ease of illustration , suppose that in the process described in the preceding paragraph , the batch i capsules contain negative red - brown electrophoretic particles and positive green particles , while the batch ii capsules contain negative yellow particles and positive blue particles . furthermore , assume a starting state for the display with both types of capsules having their negative pigments adjacent the front or viewing surface of the display . in this state , an observer viewing the viewing surface of the display will see a mixture of yellow capsules and red - brown capsules , so that ( assuming , as is normally the case in practice , that the individual capsules are made sufficiently small that the observer cannot resolve them at a normal viewing distance ), the observer will see a color intermediate yellow and red - brown , namely gold . if there is then applied to the display , a short “ positive ” driving pulse ( i . e ., a short pulse which moves positive particles towards the viewing surface ) the positive green particles of the fast - switching batch i capsules move to the viewing surface ( and the negative red - brown particles away from the viewing surface ) without much change in the more slowly switching batch ii capsules . the observer will then see a mixture of green capsules and yellow capsules , which will appear greenish - yellow . a substantially longer positive pulse will then subsequently switch the batch ii capsules , so that the observe will see a mixture of green capsules and blue capsules , which will appear blue - green . at this point , both the positive particles lie adjacent the viewing surface . reversing the driving voltage ( i . e ., applying negative pulses ) generates a new set of colors . a short negative pulse switches the batch i capsules to the red - brown state , while leaving the batch ii capsules blue , leading to a color intermediate red - brown and blue ; the actual perceived color depends on the details of the pigments , their loading and their response in the device , and the ratio of the two kinds of capsules , but will be approximately neutral ( gray ) in hue . finally , a longer negative pulse returns the display to its original state with yellow and red - brown capsules and an overall gold appearance . thus , this mixed color display may be regarded as having gold , yellow - green , blue - green , and gray extreme optical states . these four extreme optical states define a quadrilateral in the a *- b * plane of the conventional ice color space . by appropriate choice of pulse sequences , any color within this quadrilateral can be achieved . in the specific medium described , the quadrilateral will be traversed in a counter - clockwise fashion , with the “ horizontal ” legs of the quadrilateral ( corresponding to the rapidly switching batch i capsules ) being traversed more rapidly than the “ vertical ” legs . to achieve colors within the interior of the quadrilateral , it is first necessary to drive the medium to an intermediate position on one of the vertical legs and then move to the interior with a short pulse of appropriate polarity . those skilled in the art of color displays will appreciate from the foregoing description that the specific display described above does not allow control of luminosity independently of color , since the luminosity is controlled solely by the particles visible at the viewing surface at any given time . however , in the mixed color displays of the present invention , some degree of control of luminosity independently of color can be achieved by including in the electro - optic medium a plurality of third electro - optic elements ( typically a third batch of capsules ) intermixed with the first and second electro - optic elements , the third electro - optic elements being capable of being switched between dark and light optical states , with the switching time of the third electro - optic elements differing from those of both the first and second electro - optic elements . in practice , it will generally be convenient for the third electro - optic elements to have the fastest switching time of the three types of elements . obviously , the range of colors available in mixed color displays of the present invention are limited by the color ranges achievable by the constituent types of electro - optic elements . the colors at the corners of the quadrilateral in the a *- b * plane described above are intermediate in hue between the colors of the particles in the constituent capsules , so that generally very pure colors are difficult to obtain . however , the performance of the mixed color displays can be accurately predicted on the basis of the performance of displays constructed from the individual capsules ( or other electro - optic elements ) by a graphical procedure as illustrated in the example below . the following example is now given , though by way of illustration only , to show a preferred mixed color display of the present invention and the electro - optic performance thereof . the mixed color display described in this example used two different types of opposite charge dual particle encapsulated electrophoretic media , with all four types of electrophoretic particles polymer - coated by the type of processes described in the aforementioned u . s . pat . no . 6 , 822 , 782 ( which require surface functionalization of the raw pigment surface before formation of the polymer ). accordingly , the procedures for functionalization and polymer formation will be described before the procedures for forming the capsules and the final display . a 1 l erlenmeyer flask equipped for rapid magnetic stirring was charged with 600 ml of ethanol , 60 ml of deionized water , and 7 . 2 ml of ammonium hydroxide . 3 -( n - styrylmethyl - 2 - aminoethylamino ) propyltrimethoxysilane ( 53 ml of a 40 % solution in methanol , uct ) was added to the resultant mixture and allowed to stir for five minutes . raw pigment ( 60 g ) was added to the mixture and allowed to react for 15 minutes . the resultant pigment slurry was discharged into two 750 ml plastic bottles and centrifuged at 3500 rpm for 10 minutes , using an allegra 6 centrifuge , available from beckman coulter , fullerton calif . 92834 . the supernatant liquor was discarded and approximately 500 g of ethanol was added to each centrifuge bottle , and the pigment was redispersed by shaking . the bottles were then centrifuged again ( 3500 rpm for 10 minutes ), and the supernatant was again discarded . the pigment was allowed to air dry over night , and then further dried under vacuum at 65 ° c . for two hours . finally , the pigment was hand ground with a mortar and pestle to a fine powder . a 1 l erlenmeyer flask equipped for rapid magnetic stirring was charged with 240 g of ethanol , 4 g of deionized water , and 60 g of pigment . ten grams of dow corning z - 6030 ( γ - methacryloxypropyltrimethoxysilane ) was added to the mixture , which was allowed to stir for 15 minutes . the resultant pigment slurry was discharged into two 250 ml plastic bottles and centrifuged at 3500 rpm for 10 minutes . the supernatant liquor was discarded , and approximately 200 g of ethanol was added to each centrifuge bottle , and the pigment was redispersed by shaking . the bottles were then centrifuged again ( 3500 rpm for 10 minutes ), and the supernatant again discarded . the pigment was allowed to air dry over night , and then further dried under vacuum at 65 ° c . for two hours . finally , the pigment was hand ground with a mortar and pestle to a fine powder . a 250 ml round bottomed flask was charged with 60 ml of toluene , 60 g of pigment ( surfaced functionalized by one of the methods described above ), 49 . 9 g of lauryl methacrylate , and an variable amount of a comonomer as specified in the table below ( an additional amount of lauryl methacrylate is substituted if no comonomer is used ). the flask was set to rapid stirring in a 65 ° c . oil bath and purged with nitrogen . meanwhile , 0 . 6 g of aibn ( 2 , 2 ′- azobisisobutyronitrile ) initiator was dissolved in 12 ml of toluene , and added to the flask by pipette . the resultant mixture was allowed to react overnight , under nitrogen and at 65 ° c . the nitrogen feed was then removed , and approximately 100 ml of ethyl acetate was added to the flask . the resultant mixture was then split between two 250 ml plastic bottles , and approximately 100 ml of additional ethyl acetate was added to each bottle . the bottles were shaken vigorously and the polymer - coated pigment was isolated by centrifugation at 3500 rpm for 15 minutes . an aliquot of the supernatant was removed for free polymer testing , and the remainder was discarded . the pigment was washed twice by adding approximately 200 ml of ethyl acetate to each bottle , shaking vigorously to disperse the pigment , and centrifuging at 3500 rpm for 15 minutes . the pigment was air dried overnight , then dried under vacuum at 65 ° c . overnight . the specific pigments and monomers thus functionalized and polymer - coated are listed in the table below . table functionalization amount run pigment method comonomer g i - 1 dupont r - 794 ( white ) b 2 , 2 , 2 - trifluoroethyl methacrylate 1 . 73 i - 2 shepherd brown 12 b 2 , 2 , 3 , 4 , 4 , 4 - hexafluorobutyl acrylate 2 . 44 i - 3 shepherd green 223 a 2 , 2 , 2 - trifluoroethyl methacrylate 1 . 73 ii - 1 shepherd yellow 253 b 2 , 2 , 3 , 4 , 4 , 4 - hexafluorobutyl acrylate 2 . 44 ii - 2 shepherd blue 212 a lauryl methacrylate 2 . 62 the polymer - coated pigment particles thus produced were incorporated into electrophoretic internal phases ( i . e ., mixtures of pigment particles dispersed in a suspending fluid ) and these internal phases encapsulated and the resultant capsules formed into displays as described below . a 60 % by weight dispersion of each polymer - coated pigment used ( pigments i - 1 , i - 2 , and i - 3 in the table above ) in isopar e was prepared , sonicated for one hour , and mixed on a roll mill overnight . 180 grams ( ca . 1 50 ml ) of internal phase was produced by combining 11 . 25 g of pigment i - 1 dispersion , 33 . 75 g of pigment i - 2 dispersion , 90 . 00 g of pigment i - 3 dispersion , 8 . 10 g of charging agent ( solsperse 17000 , 10 % by weight in isopar e ), 0 . 41 g of surfactant ( span85 ), 29 . 70 g of kraton g - 1702 ( 3 % by weight in isopar e ), and 6 . 80 g of isopar e solvent . this dispersion was mixed on a roll mill overnight . a 60 % by weight dispersion of each polymer - coated pigment used ( pigments ii - 1 and ii - 2 in the table above ) in isopar v was prepared , sonicated for one hour , and mixed on a roll mill overnight . 200 grams ( ca . 150 ml ) of internal phase was produced by combining 100 . 00 g of pigment ii - 1 dispersion , 50 . 00 g of pigment ii - 2 dispersion , 9 . 00 g of charging agent ( solsperse 17000 , 10 % by weight in isopar e ), 0 . 45 g of surfactant ( span85 ), and 40 . 55 g of isopar v solvent . this dispersion was mixed on a roll mill overnight . internal phase batches i and ii were encapsulated separately by the same procedure . a 500 ml jacketed reactor was heated to 40 ° c . and charged with 240 g of deionized water ; 10 g of dry gelatin was then added and allowed to swell . after one hour of swelling , the gelatin solution was slow - stirred ( 100 rpm ) with an overhead stirrer for thirty minutes . separately , 10 g of acacia was dissolved in 98 . 3 g of deionized water and the resultant solution heated to 40 ° c . also separately , each of the internal phase batches described above ( ca . 150 ml each ) were heated to 40 ° c . and then added , over a period of approximately 2 minutes , to the gelatin solution . each gelatin solution was stirred ( 350 rpm ) during the addition , which was conducted by introducing the internal phase through a dropping funnel , the outlet of which was placed below the surface of the gelatin solution . after the addition of the internal phase had been completed , the rate of stirring was increased ( to ca . 750 rpm ) and the stirring continued for one hour at 40 ° c . in order to emulsify the internal phase into droplets having an average diameter of about 40 μm . the emulsions produced were diluted with 153 . 3 g of deionized water warmed to 40 ° c ., and the acacia solution was added . the ph of the mixture was lowered to 4 . 80 using 10 % aqueous acetic acid , and the stirring was continued for a further 40 minutes . the temperature of the mixture was lowered to 10 ° c . over the course of two hours , and 10 g of 50 % glutaraldehyde was added . after this addition , the mixture was gradually warmed to 25 ° c . and stirred vigorously for a further 12 hours . finally , stirring was discontinued , and the mixture was discharged into a 750 ml plastic bottle . the capsules were separated by size to yield a distribution between 30 and 50 μm diameter , with a mean diameter of about 40 μm . such a distribution can be effected by sieving the capsules for 120 seconds on a 38 μm sieve and then for 120 seconds on a 25 μm sieve to produce the final capsule slurry . the resulting capsule slurries were centrifuged , and the ph was raised above 8 . 0 by the addition of 1 % ammonium hydroxide solution . the slurries were then mixed with a polyurethane binder at a ratio of 1 part by weight binder to 8 parts by weight of capsules ; a 1 % aqueous solution of hydroxypropylmethylcellulose ( hpmc ) was added as a thickener , in an amount sufficient to provide a desired weight fraction of 0 . 002 hpmc in the slurry , and pure triton x - 100 was added as a surfactant , with a desired weight fraction of 0 . 001 . the resultant mixtures were stirred for one hour . in a separate bottle , the two slurries ( batches i and ii ) were mixed on an equal - mass basis . the three slurries produced ( containing capsules of batches i and ii , and a mixture of the two ) were separately bar coated on to 5 mil ( 127 μm ) thick indium tin oxide ( ito ) coated polyester film , the capsules being coated on to the ito - coated surface of the film . the coated films were allowed to oven dry at 60 ° c . for 15 minutes to produce an electrophoretic medium approximately 30 μm thick containing essentially a single layer of capsules ( see the aforementioned 2003 / 0137717 ). a custom polyurethane lamination adhesive doped with 1000 ppm of tetrabutylammonium hexafluorophosphate ( an additive which serves to adjust the electrical conductivity of the lamination adhesive - see copending application ser . no . 10 / 898 , 027 , filed jul . 23 , 2004 , and the corresponding international application no . pct / us2004 / 23843 ) was then laminated over the capsules , and the resultant sub - assembly was laminated a second time , via the lamination adhesive , to a segmented graphite backplane to produce experimental single pixel displays suitable for measuring the electro - optic properties of the display . the electro - optical ( eo ) response of the displays thus produced were measured in both directions over a range of applied voltages and pulse lengths . the color of the light reflected by the display was recorded as a function of time during the pulse using a rapid - scan fiber - optic spectrophotometer system ( b + w tek btc 112e thermoelectrically - cooled ccd array spectrophotometer ). the spectral intensity information was transformed to a visual ( l * a * b *) color space using well - known transformations , together with calibration information derived from the reflectivity of a known white standard . fig4 a and 4b show the results obtained from displays comprising the mixed color display of the invention containing both types of capsules ( blue / yellow and tan / green ), together with two control displays each containing only one type of capsule , fig4 a being a projection of the relevant curves on to the a * b * plane and fig4 b a projection on to the a * l * plane , and using ± 5 v drive pulses . in fig4 a and 4b ( and also in fig5 discussed below ), “ bw +” denotes the curve obtained from a display containing only the blue / yellow capsules when a positive voltage is applied , and “ bw −” the curve obtained from the same display when a negative voltage is applied . the corresponding curves from the tan / green capsule display are denoted “ gt +” and “ gt −”, while those from mixed color display of the invention containing both types of capsules are denoted “ m +” and “ m −”. the tan / green capsules had a switching speed substantially greater than that of the blue / yellow capsules . it will be seen from fig4 a and 4b that the curves for the displays containing only one type of capsules are almost linear , and thus that these displays are only capable of producing a very narrow range of colors lying on the curve between the colors of the two particles used in the display . ( it was also found that , in the specific system tested , the addition to a display comprising only a single type of capsules containing a third type of electrophoretic particle having a charge of the same polarity as one of the other two types of particles did not lead to an appreciable increase in the range of colors available .) in contrast , the mixed color display containing both types of capsules displayed substantial hysteresis in its color response , and this hysteresis allowed access to a substantial area in the a * b * color plane , intermediate the locations of the displays using the two types of capsules separately but covering a much larger range of perceived colors that were available using either type of capsule alone . fig5 shows that the two constituent capsules used in these displays have substantially different response times . ( the units of the abscissa in fig5 are units of the color difference δe * between the l * a * b * coordinates at the relevant time and the similar value after a long + 15 v drive pulse , chosen as a matter of convenience to provide monotonic changes in δe *.) when a + 15 v pulse is applied , the fast ( green - tan ) capsule takes approximately 36 msec to change half - way from the tan state to the green state ; the reverse transition requires approximately 60 msec . the slow blue - yellow capsules are several times slower , requiring 410 ms and 260 ms for the + 15 v and − 15 v transitions respectively . the relative response times for the two types of capsules are one determinant ( along with the colors of the constituent pigments ) for the width of the hysteresis curves in fig4 a and 4b . the larger the difference in response between the two capsules , the larger the area of hysteresis . the relative response times of differing capsules can be controlled by several factors , including the charge ( zeta potential ) and size of the electrophoretic particles , and especially the viscosity of the suspending fluid and the size of the capsules , with higher viscosity and larger capsules yielding slower response times . the viscosity of the medium can be controlled either by solvent choice ( e . g ., isopar e vs . isopar g ) or by polymeric viscosifiers ( e . g ., poly ( isobutylene ) or kraton ( styrene - butadiene block copolymers , also used as image stability addenda in electrophoretic media ). obviously , the methods used to control the switching speeds of other types of electro - optic elements will vary with the types of elements used . for example , the switching speeds of microcell media can be varied in ways directly analogous to those used for encapsulated electrophoretic media , while in the case of the rotating bichromal member media , switching speed can be varied by controlling the magnitude of the dipole in the rotating bichromal members . it will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the present invention described above without departing from the scope of the invention . accordingly , the whole of the foregoing description is to be construed in an illustrative and not in a limitative sense . for example , the displays etc . of the present invention may be used with any of the aforementioned types of electro - optic media . electrophoretic media , especially encapsulated electrophoretic media , tend to be the preferred media , but numerous other types of imaging media may be used . when electrophoretic are used , they may contain any known components , for example as described in the aforementioned e ink and mit patents and applications .