124 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



from x which will excite sensation, will be the ray which 

 traverses b and reaches the nerve-fibre b, while that from 

 y will affect only e, and that from x only h. The result, 

 translated, into sensation, will be three points of light on a 

 dark ground, each of which answers to one of the luminous 

 points, and indicates its direction in reference to the eye 

 and its angular distance from the other two.* 



The only modification needed in the original form of 

 the theory of mosaic vision, is the supposition that part, 

 or the whole, of the visual rod, is not merely a passive 

 transmitter of light to the nerve-fibre, but is, itself, in 

 someway 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 re- 

 garded as the physiological end of the nerve, and the 

 instrument by which the conversion of the one form of 

 motion into the other takes place ; just as the auditory 

 hairs are instruments by which the sonorous waves are 

 converted into molecular movements of the substance of 

 the auditory nerves. 



It is wonderfully interesting to observe that, when the 

 so-called compound eye is interpreted in this manner, 



* Since the visual rods are strongly refracting solids, and not empty 

 tubes, the diagram given in fig. 29 does not represent the true course of 

 the rays, indicated by dotted lines, which fall obliquely on any cornea 

 of a crayfish's eye. Such rays will be more or- less bent towards the 

 axis of the visual rod of that cornea ; but whether they reach its apex 

 and so affect the nerve or not will depend on the curvature of the cornea ; 

 its refractive index and that of the crystalline cone ; and the relation 

 between the length and the thickness of the latter. 



