DIOPTRIC MECHANISMS OF THE EYEBALL 601 



reflections of the candle from the eye at b : a bright erect image 

 from the anterior surface of the cornea ; a larger but dimmer erect 

 image from the anterior surface of the lens ; and a small very dim 

 inverted image from the posterior surface of the lens. These images 

 must be observed first when the eye at b is accommodated for a 

 distant object, and then when it is accommodated for the wire stretched 

 across the opening d. It will be noticed that the change of accom- 

 modation from far to near objects is accompanied by a change in the 

 second image (that from the anterior surface of the lens), which becomes 



FIG. 267. Diagram to illustrate principle of ophthalmo meter. (After SCHENCK.) 



smaller. The change in this image is more easily seen if the candle 

 be made to throw two images on the eye by interposing a double 

 prism at c. Then, as the lens becomes more convex to accommodate 

 for near objects, the two images of the candle reflected from its anterior 

 surface approach one another (Fig. 266). 



By measuring the size of the image of the candle produced by the 

 anterior surface of the lens, and knowing the size of the candle itself and 

 the distance from the observed eye, it is possible to calculate the 

 curvature of the lens in the living body. 



The radius of curvature of a reflecting surface is given approximately by 

 the following formula : 



R 2 a ' 6 

 R=2 



where R is the radius of curvature, a the distance of the object, b the size 

 of the image, C the size of the object. The object generally used is the distance 

 between two lights or two white objects called mires ; the ' image ' being the 

 distance between their images. Owing to the movements of the eye the latter 



