Page:Popular Science Monthly Volume 18.djvu/587

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taken by this process are perfectly clean. The plates, moreover, have the quality of preserving the impressed image for a long time after they have been exposed and before it has been developed. The chief disadvantage of the process is that the photographer using it has very little power to give local intensity to his picture. Till lately, a red light had to be employed in the developing-room, and this was painful to the operator. The difficulty has been obviated by adding iodide of silver to the emulsion in the proportion of one part of iodide to eight of the bromide, when an orange light can be used with impunity, and the shadows are given a wonderful clearness. When a collodion emulsion is adapted to a flexible support and used for the negatives, the operator is able to do away with glass and its weight, and may store rolls of sensitive material in the camera itself. Then, by turning a screw, he may place fresh portions of the band in a condition for exposure. After exposure, paper prepared with this emulsion may be moistened with turpentine, and the film bearing the image, almost free from weight and bulk, may be stripped off. To print from these flimsy negatives, it is only necessary to place them on glass. When a flexible support shall have been introduced for the gelatine emulsion, the negative processes of photography will be almost perfect. With the collodion emulsion, we are able to get a physical condition of the bromide, in which it will answer to the vibration of the rays of the lowest refrangibility. Photographs of the solar spectrum taken on this salt were shown, corresponding to wavelengths which lie more than three times as far below the red as the distance of the visible spectrum. This process is being applied to the examination of different colorless bodies, with a view of obtaining spectroscopic analyses of them. It has been discovered that the photographic image is rendered undevelopable by the action of oxidizing agents, and this alike whether it be produced on a collodion or a gelatine film or on paper. The oxidation of the image goes on also in ordinary atmospheric conditions, more especially under the influence of light, and in it we have a simple explanation of what is known as solarization. A platinum process, in which an image is produced in platinum-black, is about four times as sensitive as the ordinary silver process, and marks the greatest advance in printing that has been made for many years. Another printing process is based on the reduction, by ferrous oxalate, of bromide of silver which has been exposed to light, and was discovered almost simultaneously by Mr. Willis, of England, and Mr. Carey Lea, of Philadelphia. The prints obtained by this process have great permanence, since no organic compound of silver, the great agent of deterioration, is present in them. Mr. Lea has discovered within the last year that the power of development which is shared by most of the organic salts of ferrous oxide is not limited to them, but is possessed also by many of its inorganic compounds. He gives a brief account of his researches in this direction, and of the properties of the different salts of the oxide, in the "American Journal of Science" for June.


The Regulation of Visual Tests.—The Boston "Herald" published last summer a review of what had been done in the United States during the previous twelve months to secure protection against the dangers arising from color-blindness and visual defects in seamen, railroad-men, and other persons occupying positions of trust in which the quality of eyesight is important. Three departments of the national Government have adopted regulations on the subject. The War Department has ordered the examination of recruits by test cards, to test their power of distinguishing objects at a distance, and with worsteds for their perception of colors. The Treasury Department has made the examination of all pilots, as to their ability to distinguish colors, compulsory, with the provision that "a second visual examination will not be required in any case." The Navy Department has ordered a similar examination for all persons in the navy, or who may hereafter enter it. The House Committee on Naval Affairs has reported favorably on a petition of Dr. B. Joy Jeffries and others for the enactment of a general law of control for the naval and merchant service, and for a representation of the United States in an International Congress to agree upon definite standards of color-tests, and has reported a