MODE IN ^V^^ICH light acts on the KETINA. 281 



or papilla of the simple refracting or vertebrate eye, is speedily 

 hollowed out in front by the development in or upon it of the 

 lens and vitreous humour, so that from a spheroidal convex 

 surface, the i)rimordial protuberance assumes the fonn of a 

 cup, with its mouth directed forwards, and its cavity occupied 

 by the refracting media of the organ. Tliis cup-shaped mass 

 is the retina ; the crystalline rods are not developed on its 

 concave surface, but on its outer or convex surface, as they 

 exist on the convexity of the compound eye — that is, in the 

 direction of the radii of the sphere, but directed backwards, 

 on account of the nearly spheroidal surface. 



In conclusion, I may state what appears to be the 

 physiological superiority of the simple over the compound 

 eye. As the simple eye is acted on by reflected light only, 

 it cannot be disturbed by rays not required for the definition 

 of the image. It is also arranged so as to admit of a much 

 more delicate or minute mosaic representation of the object, 

 from its microscopic and reversed photasthetic bodies being 

 in contact with the reflecting choroidal surface on which that 

 image is formed. It moreover combines the advantages of 

 the contiguous image, formed by the lenticular structures, and 

 the mosaic image, which results from its ciystaUine rods. 



