CHAP, in.] SIGHT. 1191 



tell us that the objects are in the vitreous humour. As we shall see 

 we refer all affections of the retina, all visual sensations to some 

 changes in the external world ; and if we trusted to our sensations 

 alone in the cases of these entoptic phenomena we should suppose 

 that the causes existed outside ourselves. It is only by means 

 of inferences drawn from the features and behaviour of the 

 sensations that we arrive at the conclusion that the causes lie in 

 the vitreous humour. 



The accompanying diagram (Fig. 144) illustrates how the 

 position of objects in the eye may be determined by the move- 

 ments of their shadows on the retina. It represents the reduced 

 diagrammatic eye seen in vertical section, with n the nodal point, 

 p the principal plane, and F the plane of the principal anterior 

 focus. 1 represents an object in the anterior chamber, 2 another 

 in the substance of the lens, and 3 a third in the vitreous humour. 

 If a bright light be looked at through a pin-hole in a card placed 

 in the plane of the principal anterior focus F so that the hole 

 is at the principal anterior focus a, the rays of light may be 

 considered as diverging from a, and we may draw them as 

 refracted at the principal plane p, and then passing parallel 

 through the vitreous humour. The image on the retina in this 

 case may be represented by a'. The field of vision, limited by the 

 shadow of the iris, will be circular ; the shadow of 2 will lie close 

 to the optic axis, that of 1 a little above it, and that of 3 some 

 little way below it. It will of course be remembered that in the 

 apparent image all the features will be inverted ( 707). If now 

 the card be moved upward so that the light emanates from the 

 pin-hole at b, and the paths of the rays of light be drawn as before, 

 the image resulting will be that shewn at b'. The shadow of 2 

 has changed very little in position ; but that of 1 has moved 

 downwards, while that of 2 has moved upwards so that all three 

 lie closer together. If, on the contrary, the card be moved down- 

 ward to c the result will be that shewn in c'; the shadow of 1, 

 as before, has moved but little, while that of 1 has moved upward, 

 and that of 3 downwards, so that the three shadows are farther 

 apart. 



Thus while the shadows of objects in the anterior chamber 

 move in a direction the opposite to that of the movement of the 

 source of an illumination placed in the plane of the principal 

 anterior focus, the shadows of objects in the vitreous humour 

 move in the same direction as the source of illumination. Hence, 

 by observing the direction of the movement of an entoptic image 

 resulting from the movement of the illumination, the position in 

 the eye of the object giving rise to the image may be determined, 

 regard of course always being had to the so-called mental inversion 

 of the retinal image. Stated more strictly the rule would run 

 thus. The shadows of objects in front of the nodal point ( 705) 

 in the lens move in a direction contrary to and those of objects 



