used in photography, consists of a lighttight box having at one end an achromatic lens and at the other a ground-glass plate and a space in which a plateholder containing a sensitized plate can be placed. In order to regulate the amount of light passing through the lens and to increase the distinctness by cutting off the outside rays, the lens is provided with a series of diaphragms with different-sized openings. The camera is so constructed that the distance between its ends can be varied. By making this the proper distance the image is brought to a focus on the ground glass, then the sensitized plate is put in its place, and the exposure is made.
The plate is afterward developed by means of certain chemicals. This process reduces the silver salts in the film to a metallic condition, forming a dark layer wherever the light has acted. The salt that has not been acted upon by the light is dissolved out by putting the plate in a solution of hyposulphite of sodium, after which the plate is washed and dried. The plate is now a negative (Fig. 1), and prints can be made from it upon sensitized paper (Fig. 2)
Figure 3 shows a moving picture camera; Fig. 4, a machine for sending a photograph a long distance by wire.
IG. 539.-The Telestereograph Transmitter
By this instrument, and a receiving apparatus, photographs can be sent by wire hundreds of miles in a few minutes, and are distinctly reproduced. The transmitting record is a photograph prepared in bas-relief on a revolving cylinder. A stylus connected to a telephone transmitter traces the record, the current varying with the height of the record. This variation in current is transformed at the receiving end into various gradations of a beam of light on a sensitized cylindrical plate, revolving like the first, and from this the picture is developed by the ordinary process. The picture at the left was received in New York from St. Louis by this apparatus in November was re 1920
