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 ¢, and that from # only kh. 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. 
