Page:Popular Science Monthly Volume 90.djvu/522

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506

��Popular Science Monthly

���of one of the lenses, the eye is of long focus and the objects instead of being reproduced in sharp focus on the retina, or back of the eye, go well beyond it and produce only an indistinct impression.

A pair of under-sea eyeglasses invented by Dr. Dudgeon, an English physicist, were designed to overcome this difficulty by means of an "air-lens." This air-lens con- sists of two surfaces of ordinary glass enclos- ing air and forming certain curves. In air it has no effect at all, neither magnifying nor reducing an object; but in the water it acts as a converger, and shortens the focus of the lens of the eye.

��/iCRYSTALLlNE LENS

��RETINA-Ji

��The "mermaid" goggles in use in the picture above. At right: A diagram of the eye showing the convergence of light rays. The parts of the under-sea goggles

Eye- Glasses for Divers and Mermaids

WHY cannot the human eye focus sharply under water even when protected from it? Briefly, because the water makes the eye long- sighted. The human eye oper- ates like a camera lens receiving the light rays from the image and making them come together or con- verge in such a way that half-an-inch or less will be sufficient to reproduce a twenty- foot object.

This convergence of light rays or waves, is due to the phenomenon of refraction. It happens that the "refractive index" of the front of the eye is practically the same as that of salt water. This means that the curvature of the front of the eye is lost under water, and the rays of light instead of being deflected by that curvature, as in the air, enter straight. Thus deprived

��Taking Photographs with a "Shot-Gun" Camera

THIS gun-camera resembles outwardly the familiar "pumpgun" or repeating shotgun, with the stock, grip, trigger and slide- handle for the left hand. A small camera of the roll film sort is mounted in the large muzzle. Pressure on the trigger squeezes a bulb lying below it, which through tubing running to the shutter of the camera in the muzzle, trips it and makes the exposure. The action slide handle under the left hand is arranged to turn a new strip of film into position for each exposure, and so per- mits the operator of the gun-camera to take pictures as rapidly as he would fire a regular gun of the powder and "bang," sort. Regular sights, front and rear, are mounted on the gun to direct it accurately.

���OBJECT PHOTOGRAPHIC LENS IMAGE

���He merely presses the trigger for the exposure, and rapidly operates the slide handle to turn off a new length of film, without the necessity of removing either hand from the gun

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