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 



