264 E. J. SPITTA ON SOME EXPERIMENTS RELATING TO 



portions — the "cones" and "reds." The former are the flask- 

 like bodies much in evidence, and the rods can be seen lying 

 adjacent thereto. The point to bear in mind is that the rods 

 and cones are separated from the cornea by the lens, the 

 intervening spaces being filled by fluids of varying consistency, 

 called the " humours." It has been said already that no 

 explanation has been given as to the special use of the rods 

 and cones, and consequently physiologists are driven to regard 

 the retina, taken as a whole, as being a complicated membrane 

 in communication with the brain for the reception or the 

 perception of the image produced by the lens. We say 

 "reception or perception," because it does not seem certain 

 whether the retina perceives an image as such, and then com- 

 municates the fact to the brain, or whether it merely acts as 

 a sensitive receptive screen upon which the image is cast, the 

 real perception being due to the optic thalamus — the portion 

 of the sensorium to which the nerve fibres from it are traceable. 

 Fortunately for the present purpose, this difficult matter is of 

 no consequence, for the decision is not at issue ; but what I wish 

 to impress upon the reader is, that the cones — whatever may 

 be their function — have certainly nothing to do with the forma- 

 tion of the image, but only with its transmission. 



We now pass on to the insect's eye in section. We note, 

 first, that the retina is in direct contact with the cornea ; 

 secondly, that there is no lens proper, and we read that such 

 is usually absent, and that when it is present it only consists 

 of three or four hypodermis cells (Packman). I must here 

 remark how curious it is that so many entomologists stem 

 to mix up the cones of the retina with the true lenses, for 

 they often call them the " crystalline lenses," ascribe to them 

 quite a number of wonderful properties, and demonstrate how 

 they assist in the formation of the image produced by the 

 complete ommatidium. Yet in almost the same breath we 

 are told that the insect's eye is divided into three well-marked 

 varieties : the Eucone eye, that has a well-developed cone ; the 



