SHELLAC MOUNTANT.—Made by dissolving one ounce of shellac in four ounces of methylated spirits, and allowing to stand for some time, when the solution will be ready for use. Another method is to dissolve the shellac in a hot solution of borax and water, but the borax is liable to spot the prints.
SHUTTER.—For ordinary photographic work the lens is fitted with a cap which covers its front, and which is removed when making the exposure. For many classes of work, however, a shutter is necess ary; for although it is possible to make a very rapid exposure by remov ing and replacing the cap as quickly as possible, yet it will be under stood that exposures of tooth of a second cannot be possibly done in this manner. The shutter is, therefore, a mechanical arrangement to effect this, and a very great variety have been constructed. They may be divided into the following classes: I. (a) The simple flap shutter. (b) The double flap. (c) The flap and drop.
2. The simple drop shutter.
3. The " go and return." 4. The roller blind shutter.
5. The diaphragmatic.
With regard to the first-class it may be stated that they are of little service for very rapid exposures, and that they give a longer ex posure to the foreground than to the sky—an arrangement of great service at times when the contrast is very great, as it often is.
The simple drop shutter, when properly constructed, is capable of giving very rapid exposures, and is for this reason very often employed. 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 (see fig. 394) usually two or three times the diameter of the lens. Its rapidity can, of course, be increased by means of an elastic band or spring.
Of the "go and return " class there are two well-known forms, one of which works in the diaphragm slot and the other in front of the lens.
These are more useful for prolonged exposures, but many, however, have a tendency to shake the camera when pushed to the utmost by the spring just at the critical moment when the lens is fully opened. The consequence is, of course, a blurred image.
The roller blind class includes several well-known and tried shutters. They are perfectly free from vibration. Many of them can be used both as time and instantaneous shutters.
The diaphragmatic class includes all shutters working between the lens combinations. One of the advantages is that the position of this shutter enables smaller moving parts to be used.
Shutters are usually set in motion by releasing a catch with the finger, setting the spring or weight into action. A better system, however, is that known as the pneumatic release. It consist of an indiarubber tube attached to a piston, and having at the other end a hollow in diarubber ball. By pressing the ball air is forced through the tube, causing the piston rod to release the shutter. By this means shaking camera is entirely avoided ; the exposure can be made at any distance without exciting attention, and if the tube be long enough it is possible to photograph oneself, or be included in a group of one's own taking. The requirements of a shut ter are thus summed up by Chapman Jones : " A shutter should be small, that it may not catch the wind, light, that it may not strain the lens mount or camera front ; simple that it may be easily repaired by the user, quickly fixed to or removed from the lens, and need no change what ever in either the camera or lens." In order that the shutter may be used intelligently, it is necessary that the lengths of ex posure given by it should be accurately measured. This is usually done by photographing a body, falling at a known rate, and measuring the length of the impression. A simple plan is to have a large black circular disc, having towards its edge a white clearly-defined spot. This disc is re volved at a given speed--say, one second per each revolution. An exposure is made with the shutter, and on development the length of the image is calcu lated--if it has moved, say ten degrees of the circle, the expo sure given by the shutter was the 11a of a second. It is impor tant that the image of the moving object be as near to the center of the plate as convenient, as the edges usually get less exposure.