Stonty—Limitation of Insect Vision. 239 
to assist a fly in its wanderings about the room, or in sopping up 
its food. 
We have next to consider how this mosaic is formed. For 
this purpose let us again turn to our model. Suppose 6400 hollow 
conical funnels to be provided, each one 
“inch long. Let them be slightly more 
than the thickness of a lead pencil at 
their larger end, and tapering from this 
down to a diameter of a sixteenth of an 
inch at their smaller end. Let the in- 
; sides of these funnels be blackened so as 
‘ : jeptaltia LapbasW, to stifle any light that falls on them. Fit 
re, 2—Tas Funnersovantn. # Small lens, of one-inch focus into the 
sects bam (Didcesumaric). larger, end (of) each, and), then pack 
the funnels somewhat like the cells of a honeycomb, over the 
hemisphere spoken above, the larger ends outwards, and the 
smaller planted on the middles of the little patches that were 
marked out on the hemisphere. The little lenses will then lie on 
an outer hemispherical sheet eighteen inches in diameter. 
Let us fix our attention upon one of the little lenses, and 
consider how it operates. The light from distant objects in the 
external world would, if not interfered with, form an inverted 
image one inch behind this lens, that is, at the distance of the glass 
hemisphere, which we shall call the primary surface; but it is 
prevented from forming more than one patch of that image by 
the blackened walls of the funnel. Accordingly, only one tiny 
patch of the image, one-sixteenth of an inch across, is actually 
formed. It is formed by the light which passes the whole way 
down the funnel, and emerging at its end, falls on the surface of 
the primary hemisphere. Here it produces one little fragment of 
the inverted image, the rest of the image which the lens is compe- 
tent to form being extinguished by the blackened walls of the 
funnel. In the insect’s eye the small portion of the image that 
emerges is, no doubt, a portion of a rather indistinct image, owing 
to the very small aperture of the lens; but neither this nor its 
belonging to an inverted image is any detriment, since all the rays 
that go to form the little patch are transmitted to a single one of 
the pieces of apparatus in the insect, corresponding to the rods and 
cones of our eyes. ‘They, therefore, can result in only one of the 
SCIEN. PROC. R.D.S., VOL. VIII., PART III. 8 
