INSTANTANEOUS PHOTOORAPHY.—In the very earliest days of photography this term was applied to what would now be considered very slow work indeed. We now usually apply this term when the exposure does not exceed one second. In some cases this only amounts to the one-thousandth part of a second. This exceedingly brief exposure is usually given to the plate by means of a suitably constructed shutter.
The immense strides that have recently been made in instantaneous photography, owing chiefly to the advent of the dry-plate process, have caused photography to become useful to almost every branch of science.
To Marcy, Muybridge and Anschtitz we are greatly indebted for the strides made in instan taneous photography. These gentlemen have succeeded in photographing moving objects hitherto considered impossible to be photographed. Galloping horses, swift-flying birds, and even bullets and cannon-balls projected from guns have been successfully photographed, showing even the little head of air driven along in front of the bullet.
Both Muybridge and Anschtitz have also succeeded in making series of twenty-four or more photographs of a horse during the time it makes a single leap, and thus illustrate its every move ment. The value of these and other possibilities with the camera for artists cannot be overesti mated. Its aid to meteorologists in photographing the lightning, to astronomers in stellar, lunar, and solar photography, and to all other sciences would require a work as large as this to describe.
For the making of instantaneous pictures a large number of suitable cameras have been devised. In most of these the lens is a very rapid one, and in some cases so arranged that all objects beyond a certain distance are in focus. With an instantaneous camera a secondary image is necessary, so that the right second can be judged for making the exposure. This is usually nroduced by a finder. In making- instantaneous exposures the following tables may he useful With these tables it will be very easy to find the distance that the image of the object will move on the ground glass screen of the camera. To do this, multiply the focus of the lens in inches by the distance moved by the object in the sec ond, and divide the result by the distance of the object in inches.
Example, find the movement of the image of an object moving 5o miles per hour at a distance of ioo yards with a lens of 9-inch focus.
9 X 876 = 7,884 ÷ 3,600 = 2i inches per second.
We mast also find out the speed of the shutter required to take the object in motion, so that it will appear as sharply defined as possible under the circum stances. To do this the circle of confusion must not exceed , of an inch in diameter. We therefore divide the distance of the object by the focus of lens multiplied by ioo, and then divide the rapidity of the object in inches per second by the result obtained. This will give the longest exposure permissible in the fraction of a second. For example, we require to know the speed of a shutter required to photograph an express train traveling at the rate of 5o miles per hour at a distance of 5o yards with an 8i-inch focus lens.
The train moves 876 inches per second.
1,800 distance in inches ÷ (8+ x too) — 1,800 ÷ 850 "•- ?;• 876 X 876 speed of object per sec. ÷ 1# — — — 413 — sec.
36 Given the rapidity of the shutter, and the speed of the moving object, we require to find the distance from the object the camera should be placed to give a circle of confusion less of an inch. Multiply ioo times the focus of the lens by the space through which the object would pass during the exposure, and the result obtained will he the nearest possible distance between the object and the camera. For example, we have a shutter working at one-fiftieth of a second, and the object to be photographed moves at the rate of 50 miles per hour. How near can a camera fitted with with a lens of 8i-inch focus be placed to the moving object ? Object moving 5o miles per hour moves per second 876 inches, and in the one-fiftieth part of a second it 1 7.5 2 inches, so that— Si X 17.52 8.5 X 100 X 17.52 14,892 inches 413 yards Instantaneous photography can only he suc cessfully performed in very bright actinic light, and should never be attempted on dull days, as under-exposure will be the inevitable result. In developing it is necessary to employ a strong developer to bring up the detail. Some oper ators make use of an accelerator for this purpose, but it is not to be recommended ; the simplest is a few drops of hyposulphite of solution added to about io ounces of water. In this the plate is bathed for a few seconds previous to devel opment.