APPLICATIONS or PEOTOGRAPHY.—A number of the ordinary applications of photography have already been incidentally noticed ; but, in conclusion, it is necessary to catalogue other important uses to which photography has been applied. Its widest and most popular range of usefulness is found in ministering to the wants of all classes of society, and even providing the poorest familiea with picture galleries in miniature. But ita sphere is ever widening, and its utility being demonstrated in a thousand different waya.
Apart from portrait and landscape work, the following are among the principal applications of photography :— Reproducing works of art in permanent pigments ; reproducing maps, plans, architectural and mechanical drawings, engravings, and manuscripts, by photo-engraving, photo-lithography, and collotype printing ; photographs for book illustration, printed in permanent ink by Woodbury-type, collotype, and carbon process ; encaustic photographs, Woodbury-type, and carbon transparencies, for art and decorative purpos3s; micro-photography, by means of which, microscopic objects are enlarged for book illustration, and for educational purposes in class-rooms ; photo-micrography has been successfully employed in reducing official despatches, charts, newspapers, (Szc., to dimensiona so small as to admit of their being placed in a quill, and transported by carrier pigeons to besieged cities in time of war ; photographing columns of water raised by torpedoes ; balloon-pbotography, by which photo-surveys are obtained of au enemy's country ; astronomical photography, employed in photographing aun, moon, stars, and their spectra ; photography in the hands of chemists has proved of the greatest service in spectrum analysis, and in investigating the phenomena of inter ference of the rays of the spectrum.
Photography in colour remains to be discovercd, little or no progress having yet been made in this direction. It has been proved that certain colours of the solar spectrum may be reproduced ou a sensitive photographic plate, and that certain other colours make no impression on the fihn. Having got thus far, there can be no reasonable doubt that a polychrome system of photography will be ere long discovered—a system which shall admit of the photographing of natural objects in all their varied hues. Development may be looked for in other directions as well, in the extended
application of the art-science to the requirements of art education, and in its application to science. The day, indeed, may not be distant when photo-telegraphy may become an accomplished fact, whsn it will be possible to telegraph a portrait from one continent to another.
(See Printing and Engraving.) T. T.
PlIOTOMETRY (FR., Photometric ; GER., LiChtMeSSMag).
" Photometry " (light-measurement), or the comparison of the visible intensities of different lights among each other, is a term usually restricted to the intensity of the light in question as it affects the eye of the observer, and does not include a determination of the chemical intensity of light, which, as is well known, does not coincide with what may be called its luminosity; nor does it embrace any measurement of the intensity of the heat accompanying the light under examination.
11, fact familiar to photographers that the most sunny days are not those on which their chemicals " work quiokest," e. are most powerfully affected by light, some of the comparatively (lull days in March, for example, being more auitable for photographic work than much brighter days later in the year. This want of coincidence between the chemical, luminous, and heating maxima in the spectrum, or coloured band produced by the decomposition of white light when passed through a prism, is illustrated in Fig. 1098, which is nut, however, a representation of the spectrum of any particular light, but a typical diagram.
0 P R Q represents the viaible part of the spectrum ; the curve 0 CQ ahowa the intensity of the eye-apparent light, with a maximum over the yellow part of the spectrum ; the intensity of the heat-rays is shown by curve D E F, with a maximum just outaide the red end Q R ; while the chemical intensity is indicated by the curve A B G, with a maximum near the violet end of the visible spectrum. The heat-measurements are usually made with a delioate thermo-electrio pile, while we are indebted chiefly to Professora Bunsen and Roscoe for methods of measuring the chemical intensity of light.