310 



PHYSIOLOGY 



CHAP. 



to understand how the blurred but single image seen when the 

 retina is stimulated by diffusion -circles may split up into two, 

 three, or four images of decreasing intensity and distinctness. 



Johannes Miiller did not attempt to give an adequate explana- 

 tion of monocular polyopia. " These phenomena," he writes, " are 

 due to the construction of the eye ; in all probability they depend 

 on the different systems of fibres of which each layer of the lens 

 is composed." 



Briicke suggested that the phenomenon might be due to 

 spherical aberration of the surfaces of the dioptric mechanism ; but 

 he failed to demonstrate this, or to formulate any adequate theory. 



Bull referred it to the unequal refraction of the various sectors 

 of the lens. But this assumption was confuted by Verhoff (1902), 

 who first offered a correct explanation of monocular diplopia, 



FIG. 143. Construction to show refraction of parallel rays that pass through a lens 

 with a spherical surface. (Verhoff.) Explanation in text. 



which he found in many cases of astigmatism, by demonstrating 

 that it could be reproduced in a photographic camera with a 

 round lens, or better, a positive spherical lens uncorrected for 

 spherical aberration, arranged with a stenopaic slit. Under 

 conditions of hypermetropic refraction a double image of the 

 luminous point is formed on the screen. He accordingly ascribed 

 the phenomenon to astigmatism, associated with ordinary spherical 

 aberration of the human lens. Inspection of Fig. 143 will explain 

 this. Here the cone of parallel rays coming from a luminous 

 point at infinite distance is refracted on passing through a denser 

 medium limited by a spherical surface. The normal ray, i.e. that 

 which cuts the principal axis of the lens, is not deflected ; but all 

 the other rays are more refracted in proportion as they are farther 

 from the centre and nearer the periphery of the lens, i.e. as the 

 angle of incidence at which they penetrate the spherical surface of 

 the lens is greater. 



