Page:Popular Science Monthly Volume 28.djvu/62

54 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 moment 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 instantaneously 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 beyond 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 between 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