Friday, May 9, 2008

Insect Vision


The Probabilities as to What They Can See With Their Strange Eyes.

Two theories of insect vision are extant — the older one that each lens forms a separate eye, and the more recent one that insects see as in a mosaic, each lens forming part of the total picture. The old view is supported by the experiment of placing a thin slice of an eye under the microscope, when the image of any object reflected from the mirror is seen to be transmitted through each of the hexagonal lenses. A rough model of an insect's eye may be made by taking a glass shade of the form of a half sphere, say about 16 inches in diameter. Place the eye at the center of the globe and paint on its surface the picture of the outer world as seen through it. If this painting be divided into squares by lines scratched at intervals of one-eighth inch, there will be 25,600 of them, and the proportions are roughly those found in the eye of a dragon fly.

If now, instead of the details of the picture on each square, a dab of color be placed on it corresponding to the general tone, the effect of the whole will pretty nearly agree with that of the original painting. It is probably this indistinct vision that insects actually possess. Mathematically it may be shown that to obtain anything like such perfect vision as human beings enjoy an eye constructed on the compound type would have to be of most impracticable size. In our eyes the rays of light passing through a single lens form an image on the concave retina. The retina is built up of the sensitive terminations of the optic nerve, forming, a kind of tessellated pavement with 36,000,000 squares to the square inch. If our lens were of perfect shape and the pupil wide enough, the size of things which might be seen as distinct objects would be limited by the distance of the nerve endings of the retina from one another. In order that two points may appear separate to the eye, they must subtend an angle of about one minute of arc — that is, for instance, that fine lines ruled one-twentieth of an inch apart can be made out to be separate at a distance of four yards. Beyond this the whole surface has a uniform gray tint.

Calculations clearly show that insects cannot see nearly as well as this, and their behavior to distant objects favors this opinion. But their eyes have this advantage — namely, that there is practically no limit to the nearness of objects they can examine. The details of their own antennae probably appear plainer to themselves than to us, but objects at the distance of a foot appear to them with about the same minuteness of detail as would be attained if they were made of rather coarse wool work. Beyond this the shades of light and dark are evident to them, but outlines must be blurred and lost. — North British Advertiser.

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