fig1 shows a subjective ophthalmic refractor 10 formed in accordance with a preferred embodiment of the present invention . ophthalmic refractor 10 is of a type well - known in the art of ophthalmic instruments in that it generally comprises a right eye battery 11 a and a left eye battery 11 b that are mirror images of each other . eye batteries 11 a and 11 b comprise respective housings 12 a and 12 b and respective viewing paths 14 a and 14 b along which a patient facing a back side of the instrument gazes during examination . the construction and operation of ophthalmic refractor 10 are generally and substantially as taught in u . s . pat . nos . 2 , 968 , 213 and 2 , 995 , 065 , both these patents being incorporated herein by reference . the present invention relates to an improvement involving a cylinder power adjustment system found in each eye battery 11 a and 11 b . for sake of simplicity , the invention is described with respect to right eye battery 11 a only , it being understood that left eye battery 11 b is a mirror image of right eye battery 11 a . battery 11 a comprises a cylinder power adjustment knob 16 , a cylinder power scale 18 having numerical markings 20 indicative of a cylinder power introduced in viewing path 14 a ( see fig6 ), and an opening or display window 22 in battery housing 12 a for allowing an appropriate power value on scale 18 to be viewed by an ophthalmic practitioner . referring also now to fig4 and 5 of the drawings , adjustment knob 16 is fixed on a shaft 24 having at its opposite end a dual gear comprising a small front gear 26 and a larger rear gear 28 . larger gear 28 transmits torque to a geneva mechanism for indexing a pair of cylinder lens carriers ( not shown ) as described in referenced u . s . pat . no . 2 , 968 , 213 . in the relevant portion shown in fig4 the geneva mechanism includes a gear 31 , a driver plate 27 , a gear phasing hub 36 , a locking plate 33 , and an end cap 35 aligned coaxially on an axle stem 37 fixed to eye battery housing 12 a . larger rear gear 28 on adjustment knob 24 meshes with gear 31 , which is rotatably mounted on an axle stem 37 and has a plurality of threaded fastener holes 32 spaced about its rotational axis . gear 31 includes a radial step 34 defining a radially reduced portion received in close slidable fit within a circular opening 40 through driver plate 27 centered on a common rotational axis of gear 31 and the driver plate . likewise , gear phasing hub 36 has a plurality of fastener holes 38 corresponding to threaded holes 32 in gear 31 , and includes a radial step 42 defining a radially reduced portion received in close slidable fit within circular opening 40 . accordingly , as best seen in fig5 a plurality of threaded fasteners 44 extend through fastener holes 38 in gear phasing hub 36 and mate with threaded holes 32 in gear 31 , whereby gear phasing hub 36 can be adjusted axially relative to gear 31 . when fasteners 44 are tightened , gear phasing hub 36 securely clamps driver plate 27 to gear 31 . however , when fasteners 44 are loosened , driver plate 27 can be rotated independently of gear 31 and vice versa . consequently , an assembly technician can selectively secure driver plate 27 to gear 31 at any chosen angular orientation relative to gear 31 . gear phasing hub 36 is provided with a radially stepped axial opening 46 slidably fitting over axle stem 37 as shown in fig5 . axial opening 46 includes an internally threaded portion 47 into which a threaded shank 48 of end cap 35 is mated after the shank passes through a central aperture 51 in locking plate 33 . when end cap 35 is tightened , it serves to securely clamp locking plate 33 to gear phasing hub 36 . when end cap 35 is loosened , locking plate 33 and gear phasing hub 36 are rotationally independent of one another . therefore , an assembly technician can selectively secure locking plate 33 to gear phasing hub 36 at any chosen angular orientation relative to gear phasing hub 36 . in the preferred embodiment shown herein , end cap 35 includes a pair of spaced holes 50 for receiving corresponding prongs of a specially made adjustment tool ( not shown ) for tightening and loosening the end cap . end cap 35 is further provided with a central axial passage 57 sized for slidable fit onto axle stem 37 . small gear 26 on adjustment knob shaft 24 meshes with an internal gear 30 of cylinder power scale 18 to rotate the cylinder power scale simultaneously and in concert with the rotational indexing of the cylinder lens carriers via the geneva mechanism . cylinder power scale 18 includes a frontward - facing first side 18 . 1 on which markings 20 are applied , and a rearward - facing second side 18 . 2 . as will be understood from the foregoing description , it is now possible during assembly of eye battery 11 a to adjust scale 18 independently of driver plate 27 until an appropriate scale marking is centered with respect to display window 22 , and then subsequently secure the driver plate 27 to gear 31 by means of gear phasing hub 36 and fasteners 44 such that driver plate 27 is at its proper rotational orientation . moreover , the rotational orientation of locking plate 33 relative to driver plate 27 can be selectively adjusted and set by means of end cap 35 such that an arcuate recess 59 of locking plate 33 is centered with respect to a long peg 60 of driver plate 27 . as a consequence of the present invention , scale markings 20 are centered to a high degree within display window 22 in a fraction of the assembly time required for ophthalmic refractors of the prior art .
a subjective ophthalmic refractor comprises a cylinder power adjustment knob for simultaneously actuating a cylinder power scale and an indexable lens carrier drive assembly . the drive assembly includes a gear phasing hub designed to allow a scale marking to be centered in a display window of the refractor and components of the drive assembly to be set in proper rotational orientation as independent operations .