5 62 THE SENSES AND SENSORY ORGANS. 



spherical cornea, and Exner has not even shown that the 

 plane of the retina corresponds with the theoretical plane in 

 which such super-position could occur. 



Moreover, Exner's 'super-position theory' is admitted by 

 him only to be applicable in certain rare instances where he 

 assumes the dioptron to be an optically homogeneous vitreous, 

 an assumption which I do not think is ever justified. Exner, 

 therefore, falls back on what he terms the theory of images by 

 ' apposition,' in which he assumes that each ommateum acts 



independently. 



If Exner's view is accepted with regard to the condition of 

 the light-rays after leaving the lens or refractive cone, I can 

 see no difference in this view and Mtiller's theory of 'mosaic 

 vision.' If, however, instead of parallel pencils the foci of 

 converging pencils fall upon the receptive surface, Exner's 

 apposition theory and mine are identical, and the retina 

 receives a second image which is not inverted. 



Exner compares each ommateum to an astronomical tele- 

 scope, but he does not apparently see that if the focal lengths 

 of the refractive agents are slightly altered it is possible that 

 a second real image may fall on the receptive surface of each 

 ommateum when his theory and mine are identical. 



Exner chiefly worked with eyes in which the crystalline cone 

 is well developed, whilst my observations have been principally 

 directed to eyes in which there is no crystalline cone properly 

 so called. 



If we admit Exner's theory of the formation of an image by 

 a refractive cylinder, and suppose that the rays would leave 

 the cylinder as parallel pencils, when the cylinder is a real 

 cylinder and the first image lies midway between its ends, and 

 then add a spherical surface to the inner end of this cylinder, 

 it is possible to so arrange the curvature of this surface that 

 a real image is formed, the convex extremity of the cylinder 

 acting as a plano-convex lens (see PI. XL., Fig. 2, C, D). 



On the Other hand, in the Blow-fly imago and in the Uragon- 

 llics no such cone exists, and the subcorneal image is produced 

 by a corneal lens. If a second image is produced at all, it must 



