482 



SUMMARY OF CUKRENT RESEARCHES RELATING TO 



deviation on which the accuracy of registration depends is absohitely 

 fixed. 



The arc is of stout brass, concentric with the cornea under observa- 

 tion. It can be rotated around the polar axis of the eye so that any 

 meridian can be observed. X transparent goniometer is attached to the 

 telescope, also pointer and means of sighting to set telescope and insure 

 the images of the mires being within the field of the eye-piece. The arc 



SSls 



Fig. 77. 



carries two symmetrically moving lanterns, or " mires," which are set in 

 motion by rotating a single milled head engaging a double rack. Each 

 mire contains an electric lamp, and one has a stepped stencil, the other a 

 parallelogram with cross-line ; the latter carries a red glass and the 

 former one of green tint. The stepped mire is divided into six steps,, 

 each of which is exactly equal to one dioptre of curvature. The arc is 

 graduated with two scales, one in half dioptres from ^50 D to 60 D, and 

 the other showing the actual radius of curvature of the cornea expressed 

 in mm., from 5 • 5 mm. to 'J mm. A small mirror attached to arc admits 

 of the scales being read from the eye-piece end of the telescope. 



