Before quitting the subject of vision, I must say a few 
words, if only by way of recognition and admiration, as to the 
work that has been done at the compound-eye by anatomists. 
There is, perhaps, no object more difficult to investigate 
minutely by the aid of the microscope than the compound- 
eye of an insect. I need not refer in detail to the difficulties, 
for you all no doubt have some idea of them. These diffi- 
culties have, however, been to a considerable extent overcome 
by the skilful application of the great resources of modern 
microscopy. Grenacher, Hickson, Lowne, and Patten have 
published descriptive memoirs on the subject, accompanied 
by plates that are of the greatest value to entomologists, who, 
like myself and many of you, have neither time nor skill for 
personal research in such ways. But there still remains much 
to be done; indeed, the structure of each ommatidium,—that 
is, the part behind and connected with each facet,—is very 
complex. The details I shall not attempt to allude to, but 
there are certain points that have a special bearing on the 
functions of the organ, or aggregate of organs, that I may 
briefly mention. In the first place, they have detected 
nothing that can be looked on as an apparatus at all suitable 
for the formation and perception of a continuous picture. 
The percipient parts of the nervous portions of the organ 
are, indeed, the parts about which there is most discrepancy 
in their views. Grenacher considered certain parts that he 
called retinule, in connection with the rods, to be the per- 
cipient portions of the eye; but they are so different from 
the retina and membrana jacobi of our own eye, that they 
must have a very different function. Lowne rejected 
Grenacher’s view, and considered that a dense nervous 
structure that he styled bacilla, with the parts behind it, 
are the true—indeed the sole—percipient part of the eye, and 
he gave a figure showing the manner in which he supposed a 
picture was formed on this retina. Hickson followed with a 
memoir investigating more particularly the nervous structures 
in the more interior part of the eye, or, to speak more 
correctly, lying between the eye proper and the optic ganglion : 
these structures he found to be most wonderfully complex ; 
he rejected Lowne’s view as to the parts immediately behind 
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