vi DIOPTKIC MECHANISM OF THE EYE 287 



an Mii^lc df about 35 degrees with the optic axis of the latter. 

 'I 1 ! ic subject is asked to look fixedly at a distant point, in order 

 to exclude all play of accommodation (infra}. The observer 

 brings bis eye at the distance required for clear vision to the 

 same level as the eye observed, so that his optic axis forms much 

 the same angle as that formed on the other side by the rays of 

 the flame, with the observed eye. Under these conditions Mir 

 observer easily sees three different images of the flame in the 

 pupil of the observed eye, in the order indicated by Fig. 127, 

 when the flame is at the left. 



The image a of medium size is the clearest, with sharp out- 

 lines ; it is an erect, virtual image, reflected from a convex mirror, 

 represented by the surface of the cornea. The image 1} is also 

 erect, because it is reflected from a convex surface, namely the 

 anterior surface of the lens ; it is less bright, with less distinct 

 outlines, because only a few rays are reflected, since there is little 

 difference in the retractive indices of the 

 aqueous humour and the lens ; it is much 

 larger than that reflected from the cornea, 

 because it comes from a less convex mirror. 

 If the observer moves his eye slightly to one 

 side the image 5 is displaced considerably in 

 the same direction. This means that the 

 formation of this image is behind the pupil. a 



The third image c is inverted, and is FIO. 127. images of a candle- 



,i i n T ttamp, reflected from the 



thus a real image, reflected from a concave cornea , anterior surface &, 



mirror, formed by the back of the lens. It theieM?** 



is much less clear, because the number of 



rays reflected is less : it is smaller than the corneal image, because 



it is reflected from a mirror with a smaller radius of curvature. 



When the observer's eye moves sideways, its position in the field 



alters very little, showing that the seat of its formation lies but 



little behind the plane of the pupil. 



It is by no means easy to take ophthalmometric measure- 

 ments of the mirror-surfaces in the living eye. In calculating 

 the radius of curvature on the basis of the size of the images, the 

 refraction of the rays reflected from the mirror surfaces must be 

 taken into account. We need not discuss the devices employed 

 to overcome this difficulty by means of special ophthalmometers. 

 It is enough to cite the following values found by different 

 authors : 



Kadius of curvature of corneal surface . . . 6-852-8-151 mm. 

 anterior surface of lens . 2-900-4-09 



,, posterior surface of lens . 5-13 -8-49 



(c) The exact determination of the positions of the surfaces of 

 separation between the different refracting media of the eye, i.e. 



