referring to fig1 a driving lens mount 1 , carries the plus cylinder lens 2 . lens mount 1 is driven by motor 3 through drive gear 4 and idler gear 5 . driven lens mount 6 carries minus cylinder lens 7 . pin 8 extends from face 9 of driving lens mount 1 in a direction parallel to test axis 10 . driven lens mount 6 has a radially spaced recess 11 extending through a 90 ° arc . driving lens mount 1 has pin hole 12 and driven lens mount 6 as pin hole 13 which permit light from led 14 to be received by detector 15 when driving lens mount 1 and driven lens mount 6 are in the orientation shown in fig1 . in this orientation , the cylinder axis of the plus cylinder lens carried by driving lens mount 1 and the cylinder axis of the minus cylinder lens 7 carried by driven lens mount 6 are both vertical as indicated by the 90 ° meridians . this neutral position is used during the examination prior to conducting the jackson cross - cylinder tests . to practice the jackson cross - cylinder tests utilizing the cross - cylinder assembly of the invention , a selected correcting cylinder lens 16 is positioned between the patient &# 39 ; s eye and the assembly described above . the axis of correcting cylinder lens 16 is located alpha degrees from the 90 ° meridian . stepping motor 3 is activated by the practitioner and rotates driving lens mount 1 clockwise 90 ° plus alpha . after traveling 90 °, pin 8 drags driven lens mount 6 through an arc of alpha degrees . the axis of plus cylinder lens 2 is now parallel with the axis of correcting lens 16 and the axis of minus cylinder lens 7 is normal to the axis of correcting cylinder lens 16 and plus cylinder lens 2 . this position is used for checking power of the correcting cylinder lens and further actuation of stepper motor 3 by the practitioner provide the equivalent of &# 34 ; flipping &# 34 ; a conventional cross - cylinder lnes by counter clockwise rotation of plus cylinder lens 2 and minus cylinder lens 7 in stages of 90 ° from the position shown in fig2 . upon completion of the jackson cross - cylinder test for correct lens power , the cross - cylinder assembly is returned to the position shown in fig1 by clockwise rotation of driving lens mount 1 until the second signal of detector 15 receiving light through pin holes 12 and 13 from led 14 . actuation of stepping motor 3 to rotate counter - clockwise driving lens mount 1 through an arc of 135 ° plus alpha provides the proper orientation for conducting the jackson cross - cylinder test for alignment of the correcting cylinder lens axis . further incremental counter - clockwise rotations of 90 ° are used in conducting the test for axis orientation . if the practitioner changes the orientation of the cylinder axis of correcting cylinder lens 16 , driving lens mount 1 and driven lens mount 6 are rotated clockwise by stepping motor 3 until the second signal is received from detector 15 . all changes in axis of the correcting cylinder lens are conducted with cylinder lenses 2 and 7 in the orientation shown in fig1 . the practitioner would then realign the crossed - cylinder lenses as shown in fig3 by counter - clockwise rotation of the lens mounts through an arc of 135 ° plus the new alpha and the driven lens mount through an arc of 45 ° plus new alpha . since plus cylinder lens 2 and minus cylinder lens 7 have a net cylinder and / or sphere effect of zero diopter in the position shown in fig1 it is unncessary that the cross - cylinder assembly be removed from the test axis at any time during the refraction period . in addition , since stepping motor 3 drives the cross - cylinder lenses to proper orientation with correcting lens 16 , it is unnecessary to mechanically connect the cross - cylinder lens assembly with the battery of correcting cylinder lenses . referring to fig4 driving lens mount 101 is supported by carrier halves 116 and 117 to permit rotation of plus cylinder lens 102 carried by body 118 , which is carried by driving lens mount 101 . rotation of driving lens mount 101 is provided by stepping motor 103 and drive gear 104 through idler 105 . minus cylinder lens 107 is likewise carried by driven lens mount 106 which is similarly supported by carrier halves 116 and 117 . driving lens mount 101 has a radially spaced protrusion 119 extending towards driven lens mount 106 . similarly driven lens mount 106 has a radially extending protrusion 126 extending toward driving lens mount 101 . protrusions 119 and 126 have the same radial spacing and each protrusion extends through an arc of 135 °. thus the two protrusions leave a gap having an arc of 90 ° therebetween as more clearly shown in fig5 . led 114 and detector 115 cooperate to locate the neutral position of driving lens mount 101 by pin hole 112 in protrusion 119 . the fig1 shows spring washer 120 in detail . fig9 is an enlargement detailing the location of spring washer 120 between face 121 of driven lens mount 106 and face 122 of lens carrier halves 116 and 117 . this biasing means resists motion of driven mount 106 to prevent overrun and maintain accuracy of the critical cylinder positions . the cross - cylinder lenses function as previously described when operated by the practitioner . fig6 and 7 illustrate an alternate preferred embodiment . as shown in fig5 the structure and drive means of this embodiment is substantially the same as that of the embodiment shown in fig4 and 5 . however , protrusion 219 on driving lens mount 201 extends through an arc of 270 °, less the diameter of pin 208 , instead of the shorter arc shown in the embodiment of fig4 and 5 . instead of an arcuate protrusion , pin 208 extends from face 209 of driven lens mount 206 in a direction toward driving lens mount 201 . pin 208 is radially spaced to engage protrusion 219 which functions in the same manner as recess 11 in fig1 - 3 . instead of using the light operated system to locate the neutral position of the cylinder lenses , the embodiment of fig6 and 7 utilizes a pawl and notch construction , pawl 221 is pivotably mounted by pin 222 which in turn is mounted in carrier half 216 . spring 223 biases end 224 of pawl 221 inwardly toward driven lens mount 206 . driven lens mount 206 has a peripheral recess 225 which provides a radially extending face 226 . since driving lens mount 201 and driven lens mount 206 rotate counterclockwise when the practitioner is conducting the jackson cross - cylinder test , the pawl and recess do not interfere with such testing . however , when the practitioner desires to return to the neutral position of the cross - cylinder lenses , pawl 221 drops into the peripheral recess 225 and engages radially extending face 226 at the first opportunity . the gap in driving protrusion 219 permits an additional 90 ° clockwise rotation of plus cylinder lens 202 cancelling the effect of negative cylinder lens 207 . practitioners generally prefer to have cross - cylinder lenses in 0 . 25 , 0 . 375 , or 0 . 50 diopter powers . table i provides preferred optical parameters for cross - cylinder lenses used according to the present invention which is diagramatically illustrated in fig8 . all distances , spacings ( s ) thicknesses ( t ) and radii ( r ) are in millimeters , with a minus sign (-) denoting a radius having a vertex on the eye side of the lense . the radii noted are all cylinder radii and the glass has an index of refraction of 1 . 523 with an abbe number of 58 . 6 . table______________________________________lens radius thickness spacing______________________________________0 . 25 diopter ∞+ cyl 1 . 6 2105 . 37 s = 3 . 226 ∞- cyl . 1 . 6 - 2103 . 130 . 375 diopter ∞+ cyl . 1 . 6 1408 . 04 s = 3 . 226 ∞- cyl . 1 . 6 - 1405 . 800 . 5 diopter ∞+ cyl 1 . 6 1059 . 37 s = 3 . 226 ∞- cyl 1 . 6 - 1057 . 13______________________________________
a pair of cylinder lenses have a combined power of zero when their cylinder axes are parallel and a small cross cylinder power when one lens is rotated until its axis is perpendicular to the cylinder axis of the other lens . cross - cylinder lenses are used to determine the accuracy of the cylinder power and cylinder axis selected by the practitioner . by mounting one cylinder lens in a motor - driven ring - like lens mount having a member which engages another ring - like lens mount , a single motor can be used to drive both lenses ; select between parallel cylinder axes and crossed cylinder axes ; and rotate the crossed axis lenses as an assembly to the desired position .