Photo-Hy alography Photo-Micrograph y
Photo-Hyalography. See Hyalography.
Photo-Lithography. One of the most important of all photomechanical methods, in which a print is obtained from a negative and transferred to lithographic stone, and printed from in the ordinary way; there are also direct methods on the stone, when the image is generally obtained by a method corresponding to the first stages of the Fish Glue Process (q.v.). Transfer methods depend on the principles explained under Collotype. The transfer may be made from a collotype surface to the stone; but more usually the equivalent of a collotype surface is made on paper, and then put down on the stone.
Photo-mechanical Printing Processes. Method in which printing surfaces are produced from which impressions can be taken without light being concerned in the production of each copy. See Photogravure, Fish Glue Process, Galvano-graphy,. Daguerreotype (etching of), Obernetter's Process,
WOODBURY-TYPE, ZINCOGRAPHY, ASPHALT PROCESS, CHEMI-
graphy Collotype, Types (originating by Photography), Fili-
GRANE, LEIMTYPE, LITHOGRAPHY and LlTHO-PHOTOGRAVURE.
Photometer, Photometry. The simplest form of photometer is an upright rod; the two lights to be compared being so ad-justed that they throw shadows of equal intensity on a screen; the luminosity of the two being now in inverse relation to the square of the distance.
Photo-Micrography. The art of obtaining photographic en-largements of microscopic objects by the aid of the microscope. The chief advantage of photo-micrography is that the results obtained are free from much of the personal element which is always present with hand-drawn diagrams. And again, although the successful results are often extremely difficult and tedious to obtain, yet when obtained they are far superior to anything that can be done by hand. Any good firm microscope stand may be employed, and the draw-tube should be lined with black velvet or cloth - not the usual black matt varnish, which soon wears off. Several lenses will be required, and these should be of low or narrow-angle, and usually a will be found
sufficient, and awide-angle also. These should be ob-
tained from those makers who now manufacture lenses specially for photo-micrography. A large bull's-eye condenser and a