Rensselaer Semi-Weekly Republican, Volume 41, Number 53, Rensselaer, Jasper County, 19 March 1909 — EYES OF MEN, BIRDS AND INSECTS. [ARTICLE]

EYES OF MEN, BIRDS AND INSECTS.

Man’s eyes at rest are far focussed —will make no effort when seeing the moon pr earthly horizons. Birds* and fishes’ eyes at rest are near focussed—will, make no effort when looking at near-by worms and minnows. Man’s elastic lenses are under constant flattening compression. Imagine a rubber ball of flattened convex, lenslike shape, laid in between two disks of canvas, and the uniting edges of these cloths, stretched to a ring. They would flatten the rubber, and if relaxed it would thicken by its own elasticity. The thicker the lens the shorter its focus. For reading or threading a needle we relax the tension on the lens by .contracting a ring of muscle surrounding each lens, and then wait for the lenses to thicken through their elasticity. In fish the lens is set against the cornea (ap.proximately) short focus, and when it wants to see whether the shadowy object some feet away is a shark or a log, it pulls the entire round lens toward the retina, and gets as clear a vision as possible. Now we see why so many human beings need “spectacles” as they grow old —the elasticity of the lenses is gradually lost, just as it is In rubber. One more method of getting focus is employed by the eyemaker, which is dealt to some snakes. Their lenses, which are set near the retinas, are pushed forward after the manner of a pump piston, by blood pressure. Cheap eyes for cheap creatures. Focus regulated by excitement.

Some of the “eyes that can see in the dark” have no power of changing focus; so it makes no difference whether they get a “night edition” of the day’s doings or not. There is a prevailing “they say” opinion that birds’ sight is keener than man’s. This is probably not true, as only man and the simiae, which have “parallel vision,” possess a highly concentrated sensitive area in the retinas —the macula lutea. But birds aloft are in clearer air than man, and their eyes can change focus with remarkable speed, as necessitated by rapid flight. Birds of prey have voluntary (subject to the will) muscle as well as involuntary in their irides, and can increase the convexity of the cornea and its refractive power. Who has not wondered how a sparrow-hawk could dart through brush and trees and never turn a feather by collision? Man with his flying machines will be in sore need of bird focusing speed. We can see the single eyes of some insects without a lens, as in the locust. In viewing the house-fly we need a lens. The big, visible, bulging eyes we see are composed of thousands of unit, cone-shaped eyes bound into one compound eye each, of more or less spherical shape. Under a lens they look like glass-eyed pavement bent to convexity. Their faceted corneas are variously set in square, hexagonal or prismatic frames. Each glistening facet is the cornea lens of “a distinct, self-working eye. Their number in each compound eye is enormous. There are 50 such eyelets in each in the ant, 1400 are allowed the drone bee, and 3500 the “workers.’’ Our pet kitchen fly has 8000 chances of seeing food crumbs, the beetle over 6000, while more than 13,000 aid .the dragon-fly in his eleemosynary pursuit of the mosquito, offset somewhat by several thousand awarded the latter for a ‘sporting chance.” The hawk moth gets pictures compounded by ‘20,000 contributors. Over 25,000 window the brain of the mordella (beetle) and 60,000—50 it is claimed —contribute to the happy lives of some butterflies. —Dr. Edward A. Ayres in Harper’s Magazine.