THE THEORY OF ARTHROPOD VISION. 555 
garded as the physiological end of the nerve.’ In this form 
the original theory of Miiller has been very generally accepted. 
I have already discussed the morphological and develop- 
mental grounds which lead me to reject this view, and it 
remains for me to discuss the optical principles on which, I 
hold, it must be laid aside. 
Gottsche [201] first attacked Miiller’s hypothesis on the 
ground that a real inverted image is formed behind each 
corneal lens. The existence of such an image is easily demon- 
strated and was even known to Leeuwenhock. But various 
views have been held in relation to its significance. If we 
admit that the position of this image does not correspond with 
any receptive or retinal surface, which has been abundantly 
demonstrated, it can only be regarded as a source of radiant 
light. It has been argued by some writers that the image is 
adventitious, that every oil drop produces a similar image; but 
how this argument can avail those who believe in its non- 
existence, or assist in getting rid of a collection of real focal 
points from which light radiates in diverging pencils, is beyond 
my comprehension. The sub-corneal image cannot be thus 
lightly disregarded, as every convex facet must produce pencils 
of converging rays, just as every spheroid of oil has a focal 
image of objects behind it. Nor could such a focal plane be 
neglected if an oil drop were interposed in any optical 
apparatus. The presence of a sub-corneal image is in itself 
absolutely destructive of Miiller’s hypothesis, since all the light 
proceeding from it must consist of widely divergent pencils. So 
that the result of the presence of a lens in front of each of 
Miiller’s radial tubes would be that less light would penetrate 
it than if no such lens existed. 
Grenacher’s Hypothesis.—Grenacher, who is at present gene- 
rally admitted to be the authority on the compound eye, 
adopts Miiller’s view, with the modification already quoted 
from Huxley, that the great rods are light-transmuting organs. 
It is difficult to trace the origin of this modification: I think 
Grenacher first enunciated it in so many words, but there is 
evidence that it was in the minds of many writers before 1877, 
