An interesting application of photography is the so-called "gun camera" designed by the Royal Air Force in 1916 and used for the training of military aviators. (See Plate III., fig. I.) With this camera, which resembled in shape a machine gun, an aviator in training engaged in mock combat photographed his adversary, and on development the film showed the accuracy of his aim. High speed motion picture cameras, enabling photographs to be taken at speeds of several thousand a second are much used for the study of fast moving mechanisms.
The greatest application of photography is, however, to general recording. The use of photography for recording the movement of physical instruments is mentioned as early as 1876 in Tis sandier's handbook of photography. As a rule, any physical instrument can be made to record photographically. Thus, a galvanometer reflects a beam of light by means of a mirror, the movements of which are controlled by the current in the instru ment. If the light is allowed to fall on a photographic material which is moving, a record will be obtained. Convenient cameras for this purpose use plates allowed to fall at uniform rates or rotating drums covered with paper or film.
A very wide application of photographic recording is found in the oscillograph, which is used for many purposes in addition to its primary one of recording the wave form of intermittent or alternating electric currents.
The electrocardiograph is an oscillograph adapted to the record ing of the very weak currents produced by the heart.
The applications of photographic recording are innumerable; its use for magnetographs and seismographs should, however, be mentioned. (See fig. 4.) When the mirror is attached to a dia phragm, sound can be recorded and analysed. (See MOTION PICTURES.) In connection with science, photography is used (I) for re cording visible images; (2) for integrating radiation over long periods; (3) for detecting invisible radiation; (4) for measuring the intensity of radiation. Classes I and 2 covering the record ing of visible images whether for a long or a short time cannot, of course, be separated and cover between them by far the most important applications of photography.
Astronomical Photography.—One of the branches of science to which photography is of the greatest importance is astronomy. At the present time visual observation in astronomy has been replaced almost entirely by photography. A photo graphic plate is used in the focal plane of the telescope, and the observer's work is reduced to directing the telescope towards the object to be photographed and correcting any irregularities in the driving clock, which would produce a shift in the position of the image upon the plate.
In the case of reflecting telescopes, the photographic focus is, of course, identical with the visual focus, since mirror systems do not involve chromatic aberrations, and for this reason as well as because of their lower cost, all modern telescopes of the largest size are of the reflecting type. A certain number of refractors have been made with photographic correction of their lenses, but most of the large refractors are corrected only for visual rays, and with these photography is done on green sensitive plates through a strong yellow filter, so that the record is made by green light.
Stellar positions are now determined entirely photographically, the micrometer eyepiece no longer being used. In this connection, the astrographic chart should be mentioned. (See ASTRONOMY. ) A notable advance made possible by photography is the accurate determination of the parallaxes of the nearer stars. The number of stars observable has been greatly extended by photographic means, the loo in. reflector at Mount Wilson recording stars of the twenty-first magnitude; that is, about one million times fainter than the faintest stars visible to the naked eye. And it may be confidently anticipated that the projected 200 inch re flector, when completed and used photographically, will make a proportionate increase.
Spectroscopy.—In spectroscopy (q.v.), visual observation has also been replaced by photography. The ease, indeed, with which the ultra-violet and violet parts of the spectrum could be photo graphed has led perhaps to an undue amount of attention being concentrated on those regions. The photography of the longer wave-lengths of the visible spectrum became possible only after the discovery of the isocyanine and carbocyanine dyes, at the beginning of the twentieth century. Using the new sensitizing dyes discovered in 1931 and the following years it is possible to photograph the spectrum as far as 1,350 mil with ease. (See Plate III., fig. 6.) In the ultra-violet, ordinary photographic plates are satisfac tory down to 230 mih, but below this the absorption of the gelatine becomes serious. V. Schumann first investigated the short wave length region from 120 MA to 200 mil by means of plates pre pared with a minimum of gelatine. Recently, satisfactory photo graphs have been made in this region on ordinary plates sensitized with fluorescent bodies.