That large camera, with which the artist took your picture, was in principle just like the smaller and more simple one which I have shown you, and both are made to imitate, or in a rude way to be like the human eye.
Now, if I point this camera toward you, make it dark back of the camera, either by placing a black cloth over my head or in any other way, your picture will at once appear upon this glass which is at the back of the camera. Now the reason why I can see your photograph on this ground glass is because the rays of light which are reflected or come from your face, into this opening in the camera, have your likeness upon them, and when the light falls against this glass I can see your picture which is photographed upon the rays of light, just the same as your picture is photographed on paper. So every object about us is photographed on the rays of light and the picture becomes visible when we turn our eye, which is a small but perfect camera, so that the rays of light can go straight into our eye and the picture fall upon the back of the eye, which is called the retina, and with which this glass in the camera corresponds.
An ordinary looking-glass will demonstrate or show the same thing. This covering on the back of the glass corresponds to the black cloth with which the photographer shuts out the rays of light which come from the back of the camera. In the same way the ground at the bottom of the pond cuts off the rays from beneath, and on this account you can see the hills, or stars, or clouds reflected in the water; so also in the looking-glass, as you turn it in different directions you can see the photographs of persons or objects which are pictured upon the rays of light.
You may have thought that you saw the person or objects themselves, but this is not the case. With your eyes you can see nothing in the dark; even the cat and the owl must have some light, although they do not need as much as we, before they can see. The rays of light carry the pictures of the objects, and where there are no rays of light we can see nothing.
Now, while your photograph is being taken from the few rays of light which pass into a camera, you see that we might place hundreds of cameras one above another, and if they were all pointed at you they might each take a photograph of you at the same instant--the same as one thousand different persons in an audience with their two thousand eyes all look toward the speaker and see him at one and the same instant.