PHOTO-MICHOGRAPIIY consists in the enlargement of microscopic objects, by means of the microscope, and the projection of the enlarged image on a sensitive collodion film. The manipulatory,details are the same as in the collodion process (q.v.). only that, on account of the delicate nature of the markings to be rendered, ft is necessary to employ a collodion yielding what is termed a structureless film. The principle upon which the enlargement is effected is that of the conjugate foci. This branch of microscopic and photographic science has proved a useful aid in the study of the sciences of botany, physiology, and entomology, by delineating, with unerring accuracy, woody fiber, ducts, starch granules, musealar fiber, blood disks, nerve papillm, etc. Among the numerous experimenters attracted by this interesting study, Dr. Maddox is perhaps the only one who has attained to any renown; and by him, minute animalcule, all but invisible by unassisted vision, have been magnified to a superficial area of ti sq. in., in which the
most delicate details have been faithfully preserved. By reversing the arrangement necessary for these enlargements of microscopic objects, it will be seen that minute photographs of engravings, or other objects, may be produced which would require a microscope for their inspection. In this way communication was maintained, during the investment of Paris, when copies of letters and news vipers were inserted in quills, and fastened to carrier pigeons; and this is really by no means so difficult to accomplish as it may seem at first sight, since photographs no larger than a pin's head have been produced, including in that small space portraits of no less than 500 eminent men,