The clamps c, having each a slit of 1/2 in., can be readily moved up or down by loosening the thumbscrews.
The diagrams will show how and with what facility the shutter can be changed from one lens to another, larger or smaller. The hole in the board a should be somewhat larger than the largest tube to be used - that is, within a certain limit. For a larger class of lenses, a larger drop-shutter should be made.
An essential point to be observed in putting a shutter on to a tube, is to bring the front of the tube in almost close contact with the drop-board b - enough so not to impede the freedom of easy motion - before it is finally fastened. The inside edge of the opening of the drop-board, coming down over the lens first, must be bevelled off. The drop-board should be perfectly level. No fear of any light coming in need be apprehended, and it is not at all necessary that the tube should be encircled in wood to make it light-proof.
Where only 2 or 3 lenses are used, which are of nearly the same dimensions, the clamps can be screwed on the back of the shutter, stationary, and the slits and thumbscrews dispensed with.
All, except the board a, should be made of light wood. The drop-board b could be made of the same material as is used by some for slides in dry-plate holders - some kind of stiff and glazed cardboard, the inside painted black.
Because of lightness and probable slowness of motion, rubber bands could be employed, the same as for other drops.
Between two strips of cardboard, glued or tacked to the drop-board b, near the edges of the opening, insert a piece of cardboard, say 2 in. long, to move up and down stiffly over the upper or lower part of the opening. This makes a capital opening regulator, and will prove of great value.
With regard to lessening the speed of the fall of the drop, it needs but a little resistance to show a marked difference. The pressure of a brass spring against the drop-board, so arranged that it can be regulated, is sufficient to make the fall measure 1/4, 1/2, and 1 second. Longer exposures than one second can be better made by hand, using the cap of the lens.
In Fig. 230, a is a back view of board to go on tube; b, drop-board, with opening, running in grooves of a (see end view of a); c, clamps, made of 3/8-in. soft wood, having 1/2-in. long slits on one side; d, rubber band; e, thumbscrews passing through a and slits of c (see Fig. 231); a brass wire, on top of clamp only, prevents the rubber band from slipping off.
Fig. 231, side view of above, showing application to tube and c.
Fig. 232 shows rubber band.
The usual method adopted for this purpose depends on photographing a white clock-hand revolving rapidly in front of a black face. The chief difficulty in this case is to maintain a uniform rotation at high speed. To avoid this difficulty, and to determine the uniformity of exposure of any particular shutter under apparently like circumstances, the following method has been suggested. A tuning-fork 6, Fig. 233, with a mirror attached to the side of one of the prongs, is placed in front of the camera lens. This mirror m is so arranged as to reflect into the camera c a horizontal beam of sunlight, which before reaching the fork has passed through a 1/2-in. hole in a screen s placed about 10 ft. distant. This produces on the ground glass a minute brilliant point of light. If the fork be set vibrating, the point will become a short, fine, horizontal line; if the fork be rotated about its longitudinal axis, the line will become a sinusoidal curve described on the circumference of a circle of long radius. A photographic plate is now inserted, and the drop-shutter attached. On releasing the latter, it will be found that a portion of the sinusoid has been photographed; and the precise exposure may be determined by counting the number of vibrations represented on the plate.
The mirror employed should be somewhat larger than the lens to be measured, so as to cover its edges during the whole exposure. The mirror may be glued directly to the prong of the fork with strong carpenters' glue, after first scraping off a little of the silvering at the edges of the glass. The rate of the fork is then determined by comparison with a standard fork, by the method of beats. (W. H. Pickering.)
The necessity for carefully regulating the exposure according to the subject and the illumination is at once admitted in connection with ordinary photographic work. Whatever is true in counting by seconds, must also be true in counting by hundredths of a second; though there are many amateurs who are careful in the one case, and leave the other to luck which sometimes favours, but more often fails. And so instantaneous photography is to such operators uncertain and perplexing, except when it is confined to the same sort of views in fairly uniform weather.
It is, of course, as easy to calculate the exposure required for instantaneous as for other effects. The very beginner knows that it, for example, a view requires 2 seconds' exposure when using a certain stop and a known plate, that by changing the stop for one whose opening has an area 8 times as large, and the plate for one that is 3 times as rapid, the exposure then needed is 1/13 second. Perhaps a drop-shutter will give an exposure near enough to that calculated to secure a good picture; but on another occasion, when 1/24 second is the required exposure, the lens aperture must be reduced by a diaphragm, and the view be taken only half as instantaneously as it might have been, or else the shutter must be quickened by a rubber band or a spring, the wish being father to the thought that the exposure is thereby suitably corrected.