1883.] 



Internal He flexions in the Eye. 



477 



to a focus at a point in front of the retina, and would diverge again 

 from the point and form a blurred image on the retina. Now, will 

 this blurred image be the source of light from which rays diverge, or 

 will the retina act as- a concave mirror, so that rays from the blurred 

 image will be reflected from it so as possibly to converge again to a 

 focus which we may regard as the source of light ; or, to put the matter 

 more briefly, is the reflexion from the- retina "regular " or "irregular"? 



22. I have sought for an answer to these questions in allowing the 

 first retinal image to fall on the depression of the fovea centralis, where 

 the radius of curvature is much smaller. In this case there is no 

 perceptible alteration in the appearance of the ghost. This points to 

 the idea that the retina acts simply as a screen and not as a concave 

 mirror, and that we may regard the light coming from the first retinal 

 image (whether this be clear or blurred) as diverging from the retina. 



23. The facts recounted in §§• 19,. 20' lead me to believe that the 

 second image is the reflexion of the first. One more weighty argu- 

 ment may here be added. If the positions of the candle and ghost 

 be noted in any one position of the eye, and the candle be moved into 

 the apparent position of the ghost, then the ghost will be observed 

 in the place at first occupied by the candle : that is, the first and 

 second images of the candle on the retina are conjugate foci with 

 respect to some reflecting surface in front of the retina. 



24. The ghost is an inverted image of the candle ; therefore, its 

 physical cause, the second retinal image, must be erect, and hence an 

 inverted image of the first retinal image of the candle. The reflect- 

 ing surface is therefore concave. The possible reflecting surfaces are 

 the posterior surface of the cornea and the anterior surface of the 

 lens. 



25. A priori the cornea seems least probable for two reasons: — 

 1st. Light from the retina must pass through the lens twice before 

 returning to the retina, and calculation shows that the rays would 

 proceed finally, after leaving the lens, towards the retina, as if coming 

 from a virtual focus within the lens, 2'39 millims. in front of the 

 posterior surface. 2nd. The light proceeding outwards from the 

 retina and reflected back by the cornea would, in cases when the first 

 image was far from the centre of the field, be reflected on to the 

 anterior surface of the iris, and so would not reach the retina again. 



26. I have attempted to find whether there was any illumination of 

 the iris that would correspond to this - T but two observers have failed 

 to find light on the iris, which could not be traced to other causes. 

 The circumstances in the case were as favourable as possible, for I 

 made use of the sun as prime source of light, so that the light was 

 very bright and the pupil therefore much contracted. Again, if the 

 light is intercepted by the iris in its return to the retina,, the ghost 

 would disappear part by part, the edge of the iris intercepting more 



