876 



ENTOPTIC PHENOMENA. 



[Book hi. 



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 



Fig. 149. Diagram to illustrate Entoptical Images. 



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 3 has moved upwards so 

 that all three lie closer together. If, on the contrary, the card 

 be moved downward to e the result will be that shewn in c' ; the 

 shadow of 2, 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 



