240 Scientific Proceedings, Royal Dublin Society. 
optic nervelets being affected, and in some one definite way ; in 
other words, the whole of the light forming one patch, or rather 
speck, of the image can produce only one elementary visual 
impression in the insect’s mind. 
It will be observed that the image that is formed resembles 
rather a mezzotinto engraving, which consists of separate specks, 
than a mosaic which consists of patches of colour large enough to 
touch one another; and that it differs from the mezzotinto in that 
the specks are specks of light, instead of being, as in the engraving, 
specks of shade. 
If we endeavour to make out what provision is made in com- 
pound eyes for enabling the insect to accommodate its vision to 
varying distances of the object, we find, upon a scrutiny of the 
section of such an eye, that the arrangement appears to be one 
which gives to an insect the very singular power of adjusting 
ava-—-a------—------——=- Quter surface. 
eens Nuclear layer. 
————---- Layer of rods. 
iN Fibrils of optic nerve. 
) 
SS 
\ 
Fic. 3.—SECTION OF AN InsECT’sS EYE (DIAGRAMMATIC). 
“i 
different parts of its field of view to different distances, and 
operates in a remarkably simple way which may be illustrated 
upon our model if we add somewhat to it. For this purpose 
let a third hemisphere be provided, concentric with the other 
two, but smaller—suppose with a diameter ten inches across. 
Let the funnels which have been spoken of, and which lie between 
the outer surface and the primary surface, be made of some 
extensible material, like indiarubber, their outer ends being 
fastened to the lenses and their inner ends to threads of glass the 
thickness of thin knitting needles, and extending, as in figure 3, 
