ARTHROPODAL VISION. 57 



and admit that his evidences are at least as worthy of consid- 

 eration as those of the anatomists, especially since there is 

 amongst the latter a wide divergence of opinion concerning 

 the optical and sensorial functions of the organs in question. 

 Plateau found that the anatomists having decided that the 

 crystalline cone was a second refracting body in the optical 

 system, were divided in support of two theories. " Gottsche, 

 Leuckart, Claparede, Lemoine, Max Schultze, Harting and 

 Thompson-Lowne, though differing in matters of detail, assert 

 that the crystalline cone causes a new refraction of the rays 

 which have traversed the cornea, finally producing an image 

 on a retinal surface, or in a retinal region at a greater or less 

 depth in the group of cells below the cone ; while P. Lyonet, 

 J. Miiller, Zenker, Grenacher, Exner and Carriere, either see in 

 the crystalline cone an organ whose function is to concentrate 

 the greater part of the light rays which fall upon the cornea 

 into a beam directed along the axis of an ocular element, or 

 endeavor to prove that the presence of the cone renders the 

 production of an image impossible." (Plate 2, Fig. 8.) 



Plateau subjected both arthropods and vertebrates to com- 

 parative physiological tests very fully described in the Bulle- 

 tins of the Royal Academy, and reached the general conclu- 

 sion that the vision of the arthropod is never distinct, even 

 though distinct images may be formed, and that in many cases 

 there is little more than dermatoptic sensation, which in some 

 of them is probably intense, notably, it is said, in the 

 ordinary cockroach. The writer has, however, repeatedly 

 held a lighted match within an inch of the roach's head, and 

 in front of it as well as over it, without causing any apparent 

 alarm or motion, but on a closer approach, when the flame 

 probably touched the animal, 'it fled precipitately. This 

 experiment was tried in an otherwise unilluminated spot. 



Plateau tested many wingless insects in a piece of appa- 

 ratus which he calls a labyrinth, constructed as follows: — 

 On' the top of an ordinary table many small rectangular 

 pieces of card board were set on edge in such fashion that 



