689. A shutter is a mechanical device so arranged that it protects the sensitive plate from the rays of light entering through the lens, but is capable of being opened and closed at the will of the operator, thus allowing the rays of light to act upon the sensitive plate for as long or as short a time as the photographer desires.

690. There are a great many of these mechanical arrangements constructed in a very great variety of forms. They may be divided into the following classes:

(1) The drop shutter.

(2) Between-the-lens shutter.

(3) The focal plane shutter.

(4) Behind-the-lens shutter.

691. The simple drop shutter when properly constructed is capable of giving very rapid exposures. It is usually a plate of wood, metal or ebonite, which falls by gravitation in front of the lens. The center of the falling piece has an aperture of a certain length, usually two or three times the diameter of the lens. Its rapidity can, of course, be increased by means of an elastic band or string.

692. The between-the-lens shutter is composed of a great many varieties, styles and makes, some of the more simple of which we have described in Volume I., for instance, the Wollensak Regular and Automatic Shutters. As the principles of these have been very clearly defined, it is not necessary to go into further detail in this volume regarding them, further than to say that the regular shutter has to be set before each exposure is effected, while the automatic shutter sets itself after each exposure, so by merely pressing the bulb or moving a lever another exposure can be effected. The speed of the shutter is regulated by the position of a milled disc, on which is an indicator. To set the shutter for the desired exposure the disc is revolved until the indicator is opposite the proper figures on the dial. The regular is made in six sizes, and is one of the best types for general use on hand cameras. It is easy of adjustment, and even in the largest size releases smoothly and without vibration. This type of shutter consists of hard rubber leaves, working between the cells of doublet lenses, or in front of single lenses, their movement being controlled by small levers and springs.

693. The same principle of the above mentioned Iris diaphragm shutters is carried out in the Goerz X L Sector Shutter (See Illustration No. 49), and B. & L. Volute Shutter (See Illustration No. 50), both of which are much more efficient than the Wollensak, and their greater cost is due to the fact that their mechanical parts are of higher efficiency. In addition to this the "Sector" and the "Volute" give practically accurate speeds up to 1/150 of a second, while the Wollensak Shutters will not give speeds shorter than 1/100 of a second.

694. Pneumatic Release

Pneumatic Release. The pneumatic release consists of a rubber tube attached to a piston, at the other end of which is a hollow rubber bulb or ball. Pressure upon this bulb will cause the air in the tube to be forced through it, causing the piston rod to set the shutter free. In this way the camera is not moved during the exposure, and the operator can be at any distance from it. If the tube is long enough, it is possible to photograph oneself or be included in the group which he is taking.