ARTHROPODAL VISION. 55 



In 1880 Huxlej' published his memorable monograph on 

 the cra^'fish, in which the structure of the compound eye of 

 this animal is treated with characteristic thoroughness. He 

 reached the conclusion that the explanation of vision with 

 compound eyes would be found in a modification of Miiller's 

 theory, the modification suggested being this : Since the pyr- 

 amids through which the rays collected by the corneules must 

 pass are not empty tubes, but are solid, refracting " rods," we 

 may suppose that a part or the whole of this visual rod " is 

 not merely a passive transmitter of light to the nerve fibre, 

 Ijut is itself in some way concerned in transmuting the mode 

 of motion, light, into that other mode of motion which we 

 call nervous energy-. The visual rod is in fact to be regarded 

 as the ph^'siological end of the nerve, and the instrument by 

 which the conversion of one form of motion into the other 

 takes place." 



Lubbock supposed that the cr>'stalline cone in most cases 

 destroyed the corneal images, and he stated positively that 

 such is the case in Lampyris spleiididula, to which subject we 

 shall have occasion to refer later in connection with an exam- 

 ination of Exner's researches. Lubbock also warns us that 

 " we must not attach too much importance to the mere pre- 

 sence of an image. Any lenslike object, even a globule of 

 fat, will give one. Moreover, as Miiller and Helmholtz have 

 shown, the lens of the cornea would be an advantage in the 

 theory of mosaic vision by assisting to condense the rays of 

 li"ht in the termination of the nerve." 



the vitreous humor of vertebrate?) is a conical tube — one for each facet 

 with sides and bottom lined with pigment. These tubes converge to the 

 optic gangUon, the fibres of which pass through the tubes to the cornea. 

 Vision by such a compound eye is not a mosaic ; but each ocellus gives a 

 complete image, although a different perspective from its neighbor. 

 The multiplied images are reduced to one mental stereoscopic picture on 

 the^principle of single vision in ourselves." — Comparative Zoology, 

 Structural and Systematic, by James Orton, A. M., Professor of Natural 

 History in Vassar College. New York, 1876. 



