238 Scientific Proceedings, Royal Dublin Society. 
each patch be removed, and a single dab of paint substituted, of 
a tint and brightness which is the resultant of the part of the 
picture which fell within the patch. In this way, a somewhat 
coarse mosaic is substituted for the more perfect picture of the 
external world previously drawn. This coarse mosaic gives a 
rough imperfect representation of the external world, and repre- 
sents correctly the vision which an insect has of it. The compound 
eyes of some insects, especially insects that attack other insects, 
have more numerous facets than what correspond to 6400 over a 
hemisphere; and in such cases, the mosaic is less coarse, and the 
vision is proportionately better. Thus the eye of a dragon-fly is 
better represented by substituting smaller patches, each the size of 
a square eighth of aninch. This increases the number over the 
hemisphere to 25,600. But there is a somewhat narrow limit to 
improvement in this direction, owing to its necessitating a diminu- 
tion of the aperture of the lenses. The way that nature deals 
with this difficulty is by increasing inordinately the size of the 
compound eye of the insect out of proportion to its other features. 
In this way the number of the patches, one of which is formed 
by each facet, can be increased without diminishing too much the 
aperture of the little lenses. 
With a mosaic such as is described above, the diameter of each 
patch subtends about a degree and eight-tenths (1°-8) at the centre 
of the hemisphere. Accordingly, the interval between two objects 
in nature would need to subtend an angle ofabouta degree and 
three-quarters at the insect’s eye, to be distinguishable as two by 
the insect. Hence, if as far off as ten inches, the distance at 
which we see most distinctly, they would need to be separated by 
nearly a-third of an inch to be seen by the insect as more than one 
object; while, if close to the insect, only one-tenth of an inch off, 
the separation would need to be about the same as that which the 
human eye is capable of distinguishing at a distance of ten inches- 
Thus, the insect cannot see more detail upon its own antenna, 
close as they are to it, than we can with our naked eye. We 
must, therefore, dismiss from our thoughts the mistaken impres- 
sion that insects see very minute objects far beyond human vision. 
On the contrary, their vision is imperfect compared with ours. 
Still, it is evidently quite enough to enable a bee to be guided in 
its search after honey by the markings upon a flower, or effectually 
