54 THE POPULAR SCIENCE MONTHLY. 



In modern photography that which answers to the retina of the 

 eye is called a " sensitive plate." It is a small plate of glass, coated 

 with a chemical solution, so sensitive to light that it darkens the mo- 

 ment it is exposed to the faintest ray of sunlight. Let us take one of 

 these plates and, with due precautions, carefully put it in the camera 

 exactly where in the human eye the retina is situated. The camera, 

 or dark chamber, is covered in front exactly as though the eye were 

 closed ; not a ray of light under any circumstances is yet permitted to 

 enter it. 



Now comes the mysterious part of its execution. Let us suppose 

 that a man blindfolded, and this artificial eye, a photographic camera, 

 are set down in the open air in the bright sunshine before the scene of 

 some great ceremony — a procession of a thousand persons, the moving 

 panorama of a city street, or a wide extent of landscape. Suppose 

 that, the bandage being removed, the man were instructed instantane- 

 ously to open and shut his eyes as quickly as possible, and then to 

 describe what he had seen in that twinkling of an eye. What would 

 be the result ? 



Try the experiment yourself. Go to the window, with your eyes 

 closed. Open and shut them just as quickly as possible, and then try 

 to describe what you have seen in that time. It will be very little, 

 besides that which you remember from previous familiarity with the 

 scene. For the most part there will be nothing bej'ond a confused 

 idea of light and shade. The time of this momentary vision will be 

 too short to enable the human retina to perceive or the human brain 

 to register any definite impression of anything. 



How is it with the photographic camera and lens, our artificial 

 eye ? We will suppose that everything is in readiness, that its retina 

 or sensitive plate is in perfect condition, and that not a ray of light 

 has yet entered within the darkened chamber. Instead of being " the 

 twinkling of an eye," we shall arrange so that the time elapsing be- 

 tween the opening and closing of the artificial eyelid shall be less than 

 one tenth of a second, or far less than the time necessary for our eyes 

 to open and shut. It shall be as nearly " instantaneous " as possible. 

 Everything is ready. Click! It has opened and shut. What has it 

 seen in that little instant of time ? 



If anything is in motion, it has been perceived in that fragment of 

 a second as if motionless. Men walking along the street are pictured 

 with uplifted feet. A trotting-horse may be caught with all of its four 

 legs in the air, viewed just at the moment when he was clear of the 

 ground. A man leaping with a high pole may be pictured in mid-air, 

 precisely in the position in which he appears at the highest altitude. 

 Motion seems rest. 



But this is not the most wonderful of its powers. Far beyond the 

 keenest of human vision is its range of sight. If the light is good, 

 this sensitive plate of glass will have recorded and discerned a thou- 



