The Book of Stars: Being a Simple Explanation of the Stars and Their Uses to Boy Life

by A. Frederick Collins

Photographing the Stars.—There are some interesting things you can do with an ordinary camera in the way of photographing the stars.

A camera is made up of three principal parts, and these are (1) a convex lens, which forms the image of the object to be photographed; (2) a light-tight box, which keeps out all the light except that which passes through the lens, and (3) a plate, or film holder, which holds the sensitive plate, or film, in the camera and keeps it perfectly dark until you are ready to have the image formed on it.

The instant the lens is allowed to form an image on the sensitive plate, or film, we get out of the optics of photography and into the chemistry of it.

The sensitive plate, or film, is coated with a thin layer of salts of silver and bromide, heated with gelatine and a little water, and when these substances are thoroughly mixed, an emulsion results, and this is spread on glass plates or on celluloid films. When the plate is exposed, that is, when the image has been formed on it, it is developed. The developing is done by placing the plate in a solution of pyrogallic acid, or hydroquinone and water.

The plate, or film, is now put in the fixing bath, which is simply a solution of hyposulphite of soda and water. This fixing bath prevents any further action of the light on the plate, or film.

But the picture on the glass plate, or celluloid film, looks all same like a Chinese chromo, for the light and dark parts are just the reverse of that of the object which was photographed; in other words, where the picture should be white, it is black, and where it should be black, it is white, and this is the reason it is called a negative.

Fig. 180.—Camera Pointing to North Star.

To get a picture of the object, as it looks to the eye, a sheet of paper, also coated with salts of silver, is laid flat on the negative and held close to it in a printing frame when it is exposed to the light of the Sun.

When the paper is printed dark enough by the Sun, it is toned to give it a pleasing color and fixed to make it last a long time, and when washed and dried the image of the object, true to life, is there, a wonderful record for all time, nearly.