MODE IN WHICH li(;ht acts on the ketina. 279 



and probable mode of action of the photaesthetic bodies affords 

 the basis of a hypothesis which meets all the conditioms of 

 the question, and is in full accordance with the comparative 

 anatomy and development of the organ of vision. I cannot 

 interpret the functions of the structure of the retina as now 

 determined, except by assuming that the phota-sthetic columns 

 are impressed not by the light as it enters the eye, or as it is 

 more or less irregularly reflected and dispersed in its interior, 

 but only by those mys which, in their passage backwards to 

 the pupil, pass along, or nearly in, the axes of the crj'stalline 

 rods or columns of the phota^sthetic bodies, so as to reach the 

 phota?sthetic spots under the required conditions. No con- 

 fusion, therefore, can result from the multitude of convergent 

 and divergent rays which pass through the chamber of the 

 eye, and through the retina. By this means, the numerous 

 rays not necessary for vision, are as it were eliminated from 

 the operation, the eye being blind to them, and affected only 

 by such as are reflected backwards to the pupil along the axes 

 of the crystalline columns. 



2. The Crystalline Columns of tiie Conqyo^ind Eye. — As 

 stated in my lecture on the retina, formerly alluded to, I 

 conceive the crystalline columns in the eye of the insect or 

 crab to act in the same manner as the retinal rods in the 

 spheroidal or simple eye. That they do so may be held as 

 established by the researches of J. ^lUller on the laws of 

 vision in the compound eye. Miiller even refers to the 

 columnar structure of the retina, as presenting a certain 

 similarity to the structure or an*angement of the compound 

 eye. F. Leydig, in an elaborate memoir published in Miiller's 

 Archiv. in 1855, on the structure geneiully of the .iVrthropoda, 

 examines minutely the structure of the simple and comix)und 

 eyes, and arrives at the conclusion that th<? crystalline columns 

 of their lompound eyes, as well as the coiTesponding stmctures 

 in their so-culled simple eyes or ocelli, are of the some nature 



