you will generally see a ground colour of grey, blue, brown, or black; but when viewed at certain angles in a strong light the most gorgeous hues of metallic brilliancy—gold, copper, and bronze—are to be observed. All such colours are due to the reflection of light from the colouring matter that lies between the numerous conical compartments.
Fig. 4.—Section of the Eye of an Insect.
A glance at the section of a compound eye will show you that all the little cones radiate from a common centre. And, as each little compartment is surrounded by opaque colouring matter, it is clear that perpendicular rays only are capable of penetrating to its base and exciting the nerve fibre that lies there. Thus each little division of a compound eye forms its own image of the object that happens to be exactly opposite its facet. But how many facets do we find in a single eye? Sometimes only a few hundreds, but sometimes as many as seventeen or eighteen thousand! We must not, however, conclude that the nature of the vision of butterflies and moths is necessarily very different from our own. We have two eyes, but the images formed by them are both blended, so that we do not see double. We can understand, therefore, that the thousands of images formed in a single eye may be blended together so as to form one continuous picture. Still there remains this difference: while in our own case the two images formed by the two eyes are practically the same, in the case of insects every one of the little conical tubes of a compound eye forms an image of an object that cannot possibly be formed by any one of the others. Thus, if the lepidopterous insect sees a continuous picture of its surroundings, such a picture is produced by the overlapping and blending, at their edges, of hundreds or thousands of distinct parts.
There is yet another interesting difference between the vision of these insects and that of ourselves. As already stated, our two eyes are both turned toward the same point at the same time. But look at the butterfly's eyes. Here are no movable eyeballs, and the two eyes, placed as they are at the sides of the head, are always turned in opposite directions. The corneæ, too, are very convex; and consequently the range of vision is vastly wider than
ours. A boy is often easily surprised by a playmate who approaches him stealthily from behind, but did you ever try the same game with a butterfly? I have, many a time. After getting cautiously so near to a butterfly at rest as to be able to distinguish between its head and its hinder extremity, I have quietly circled round it so as to approach it from behind, being at the time under the impression that it wouldn't see me under those circumstances. But not the slightest advantage did I derive from this stratagem, for the position and construction of its eyes enabled it to see almost all ways at once.
In addition to the two compound eyes, the Lepidoptera, or at least most of them, are provided with two small simple eyes; but these are generally so hidden among the closely set hair that covers the head, that it is doubtful whether they are of much service as organs of vision.
