as shown in fig1 - 3 , a device formed in accordance with the present invention may include a housing 11 , which , if desired , may be formed in a plurality of sections which are joined together by suitable means such as bolts or screws 13 . upon one portion of the housing , a motor 15 may be mounted from which a suitable shaft 17 ( fig2 ) may extend . for the sake of convenience , motor 15 will be described here as a &# 34 ; stepper &# 34 ; motor , but any motor and / or its drive system which can be controlled for incremental rotational movement may be employed . the shaft 17 may support a square or other shaped drive shaft 19 , as shown in fig1 which may be fastened to the motor shaft 17 by an suitable means , such as a set screw 21 . the drive shaft 19 may be inserted through a complementary bore 22 in the center of a cartridge carrier 23 . the shaft 19 and carrier 23 may be fastened together by any suitable means such as a nut 25 . the cartridge carrier 23 is preferably manufactured in such a way as to support a transparency cartridge 27 thereon . although this may be accomplished in any desired fashion , as shown in the drawings the carrier 23 may employ a plurality of gripping elements 29 which , if desired , may be flexibly depressable to a position even with or radially inside of the periphery of the carrier . such gripping elements may cooperate with a like number of apertures 31 in the cartridge 27 to hold the cartridge to the carrier for rotation therewith when the motor 15 is actuated . as illustrated in fig1 the cartridge 27 may carry a predetermined number of transparencies 35 located about the periphery thereof for projection onto a viewing screen . a projection lens tube 37 may be mounted in one wall of the housing 11 . if desired , the lens tube may be threaded into a bore 38 in the wall of the housing and be provided with a knurled surface 39 so that an operator can adjust the position of the projection tube to insure the best possible resolution of an image projected onto a viewing screen 40 . as shown , the projection tube 37 may include a plurality of lenses 41 maintained at selected distances by spacers 43 , all of which may be held within the tube 37 by any suitable means such as a snap ring 45 . the housing 11 may also support an easily removable cap 51 which may be fastened in place by means of a pair of slide fingers 53 . the easy removability of the cap 51 will allow an operator to quickly and simply replace the cartridge 27 mounted on the carrier or support 23 . if desired , the cap 51 may suitably support a projection lamp socket 55 which may pass through a conical spring washer 57 and a socket bore 59 in the cap 51 . the socket 55 may be fastened to the cap by means of a holding plate 61 which , in turn , may be fastened to the cap by means such as bolts 63 , thus holding the lamp socket in a fixed position relative to the cartridge . the mounting of the lamp socket in its position on the cap is , of course , merely exemplary and any similar system may be employed . for example , it may be desired that the socket be adjustable relative to the cap so that a projection lamp 67 may be accurately located relative to a condenser lens pair 69 and a reflector 71 . lenses 69 and reflector 71 may , if desired , be mounted on the cap 51 by means such as brackets 73 and 75 . also , if desired , a lamp shield 77 may be suitably located over the lamp 67 by flexibly gripping the socket 55 . such a shield will prevent light emanating from the lamp from traveling in any direction other than through the condenser lens pair 69 , a transparency 35 aligned with the lens , and the projection tube 37 . in this manner , it will be realized that the cartridge 27 will rotate with the carrier or support 23 and stepper motor 15 to position the tranparencies 35 within the projection system including the lamp 67 , the condenser lens pair 69 , and the projection lens tube 37 . of course , the present invention relates to such a device in which a distinct , preselected transparency 35 is so aligned without requiring such alignment to occur merely as a result of continuous , sequential alignment of all of the transparencies . for this purpose , the present invention requires that some structure be provided for determining a home or starting position for the cartridge and motor , as that term has been previously described . one such structure might include , for example , a light emitter 83 and a detector 85 which may be positioned to sense light reflected from a small reflective patch 87 on the surface of the cartridge . of course , the detectable element could be located on carrier 23 , if desired . this detector and emitter pair might , for example , be connected to a printed circuit board 89 which may be connected to suitable control apparatus through a connector device 91 . with such structure , each time a new image is to be projected onto the screen 41 , the control system , including the circuit board 89 , can be used to order the motor 15 to rotate at high speed until the home position indicated by the patch 87 is sensed by the emitter - detector pair 83 , 85 . subsequently , the motor 15 must then move or step through a number of descrete angular measurements until the proper transparency is aligned with the projection system . this can be accomplished , for example , by providing a series of apertures 95 about the periphery of the cartridge 27 or carrier 23 , each aperture corresponding to one discrete step of the motor . a detector 101 may , if desired , be mounted on the end cap 51 so as to extend into the cartridge to the position illustrated in fig2 . detector 101 and motor 15 may both be connected to control system 89 , etc . in this one way , the detector 101 will sense pulses of light as the rays from emitter 83 pass through each consecutive aperture 95 . in other words , if the control system 89 orders the motor 15 to align the transparency at the 15th position on the cartridge 27 , the motor will turn rapidly until the patch 87 is aligned with the emitter detector pair 83 , 85 . then , the motor will move through discrete steps , each step being counted by the emitter - detector pair 83 , 101 as light passes through the aperture 95 . when 15 steps have thus been counted , motor movement will stop and the proper transparency will have been aligned with the projection system . of course , depending upon the characteristics of motor 15 , this homing and counting may be accomplished very rapidly so that any delay is nearly imperceptible to one viewing the screen 40 . at times , it may be desirable or necessary to provide apparatus which will indicate the identity of the cartridge 27 to the control system and / or to some remote readout structure . this is particularly important when a large number of cartridges or when several cartridges having a large number of transparencies may be employed with the device . any desired structure may be employed to accomplish this result such as , for example , an emitter - detector pair similar to that employed for either the homing or step counting as described previously . in order to exemplify another structure which could also be employed for this purpose , however , there has been illustrated a circuit board 109 to which a plurality of flexible sensing fingers 111 may be suitably connected . the board 109 may also be connected to the control system 89 , etc . in order that the fingers 111 may sense the identity of the cartridge 27 , the latter may be provided with one or more material strips 113 which may be aligned with the fingers 111 when the cartridge is in place on the support or carrier 23 . the strips , for example , might be electrically conductive metal strips or may merely be raised beyond the periphery of the cartridge a sufficient distance to cause the fingers 111 to close electric contacts on the board 109 . fingers 111 may be used to sense the presence of none , one , or several various combinations , or might be used to sense different conductivity characteristics of various strips on one cartridge as compared to other cartridges . in any event , use of this or any similar structure may be employed to present a signal to the control structure so that the latter will not seek to produce an alignment with the projection system of a transparency which may not exist on the cartridge presently in the machine . further , the structure may be used to reveal a programmed control structure that the desired transparency is located at a certain step on the mounted cartridge , although it may be in a different step on another cartridge . thus , those skilled in the art will now realize that a device embodying the present invention may employ a wide variety of structures which may be utilized to control the precise position on a cartridge mounted for rotation by a stepper motor , such cartridge being used , for example , to align a transparency with the projection system . use of a device in the nature of that described does not require that the cartridge be positioned on its carrier in some critical relationship therewith if the detectable elements are on the cartridge rather than the carrier . the homing system may be utilized to bring the cartridge to the home position regardless of the location in which the homing indicator , such as reflective patch 87 , is initially mounted relative to the carrier . the electrical system for controlling the operation of the mechanical apparatus shown in fig1 and 3 and described above is illustrated on a block diagram basis in fig4 . the electrical system includes a clock generator 200 which generates clock signals at a particular frequency . the clock signals from the generator 200 are introduced to an &# 34 ; and &# 34 ; gate 202 , as is the output from a comparator 204 , which may be constructed in a conventional manner . the output from the &# 34 ; and &# 34 ; gate 202 is in turn introduced to a phase driver 205 which provides signals to windings 206 and 208 in the stepper motor 15 ( also shown in fig1 ). the signals representing a location address are produced on lines 210 and may be in binary form . although two lines are shown in fig4 to provide the location address , it will be appreciated that a multiple number of lines may be provided , particularly when the address is in binary form . the signals on lines 210 pass to an address register 212 , which may be constructed in a conventional manner . a &# 34 ; load command &# 34 ; signal 214 is also introduced to the address register 212 . the output signals from the address register 212 pass to input terminals of the comparator 204 . an electronic latch 216 , such as a bistable flip - flop , also receives signals at one input terminal from the load command line 214 . the latch 216 also receives signals at a second input terminal from the collector of a phototransistor 85 ( also shown in fig1 ), the emitter of which may be grounded . the signals on one output terminal of the latch 214 are introduced to a reset terminal in a counter 220 having another input terminal connected to the output of the &# 34 ; and &# 34 ; network 202 . output signals are introduced on a plurality of lines from the counter 220 to input terminals of the comparator 204 . the counter 220 may be constructed in a conventional manner . when a particular tranparency is to be selected on a cartridge disposed on the carrier 23 ( fig1 ), the signals representing the address of this transparency are introduced to the address register 212 . when a &# 34 ; load command &# 34 ; signal is introduced on the line 214 , the signals representing the address of the transparency become stored in the address register 212 . the address register in turn introduces these signals to the comparator 204 for comparison in the comparator with the signals from the counter 220 . when the comparator 204 indicates that the count in the counter 220 does not correspond to the number preset into the register 212 , it introduces a signal to the &# 34 ; and &# 34 ; gate 202 to activate the &# 34 ; and &# 34 ; gate . this causes the clock signals from the generator 200 to pass through the &# 34 ; and &# 34 ; gate 202 to the phase driver 205 . these signals cause the stepper motor to operate so that the rotor in the motor steps through successive increments upon the introduction of progressive clock signals from the generator 200 . the load command signal provided on the line 214 also causes the latch 216 ( or flip - flop ) to be triggered to the set state and causes a signal of a particular amplitude to be introduced from the latch 216 to the counter 220 . this signal causes the counter 220 to be reset upon each introduction of a clock signal to the counter 220 from the &# 34 ; and &# 34 ; gate 202 . in this way , the counter 220 is reset to &# 34 ; zero &# 34 ; upon each passage of a clock signal through the gate 202 . this prevents the count in the counter 220 from corresponding with the number preset into the register 212 . as a result , signals continue to pass through the &# 34 ; and &# 34 ; gate 202 to the windings 206 and 208 in the stepper motor 15 until the patch 87 ( fig1 ) is aligned with the emitter - detector pair 83 and 85 . upon the alignment of the patch 87 with the emitter - detector pair 83 and 85 , the photo transistor 85 becomes conductive . this causes the latch 216 ( or flip - flop ) to become triggered to the reset states . as a result , a signal is no longer introduced from the latch 216 to the counter 220 to reset the counter . thereafter , the clock signals passing through the &# 34 ; and &# 34 ; gate 202 cause the count in the counter 220 to become progressively increased . when the count in the counter 220 corresponds to the number preset into the address register 212 , an output signal is produced by the comparator 204 . this output signal closes the &# 34 ; and &# 34 ; gate 202 and thereby prevents any further clock signals from passing through the &# 34 ; and &# 34 ; gate . in this way , the stepper motor 15 steps the cartridge to the transparency represented by the number preset into the address register 212 . when this transparency is reached , the &# 34 ; and &# 34 ; gate 202 becomes closed so that the stepper motor 15 cannot be further energized . means may also be included in fig4 for identifying the particular cartridge to make certain that it is the one which is desired to be processed . for example , a register 230 may be included to identify in digital form the cartridge indicated by the pattern of signals from the sensing fingers 111 . a register 232 may also be provided for identifying in digital form the cartridge manually selected on the basis that it contains the desired information . the signals from the registers 230 and 232 are introduced to a comparator 234 which is constructed in a manner similar to the comparator 204 . the comparator produces an output signal when the patterns of signals from the registers 230 and 232 are identical . the signal from the comparator 234 is introduced to the &# 34 ; and &# 34 ; network 202 so that the &# 34 ; and &# 34 ; network will be activated only when the desired cartridge is being processed . the present invention thus provides an alignment and indication system which is rapid in operation , inexpensive to produce , and very simple and non - complex for construction and maintenance . its features may be employed in a wide variety of structures which may not physically resemble that described here but which , nevertheless , employ the present invention as set forth in the following claims .