Page:Popular Science Monthly Volume 22.djvu/419

Rh which light is allowed to fall on a sensitive plate, and is intended as a guide to determine the comparative rapidity of the plates. Mr. Warnerke has also introduced an actinometer, or instrument to measure the intensity of light, which is dependent on phosphorescence for its value. It consists of a phosphorescent tablet, by the exposure of which to the action of light he is able to tell the photographic value of the particular light. The discovery is of the more value, because phosphorescence is induced by very nearly the same rays as those which affect bromide of silver. Another simple way of telling the amount of exposure to give the plates is by Woodbury's photometer, in which a piece of bromide paper exposed to the light is compared and read off with one of a series of tinted circles. A rule to be remembered in using this instrument is, that if a bromide plate is used, a bromide paper only should be used for securing the tint; if a chloride plate, a chloride paper. Recent researches of mine have shown that the darkening intensity and the developing intensity go hand in hand; therefore, when the operator has the number which gives the right tint, he may always be sure of getting the right exposure.

Some of the most recent and striking exemplifications of the scientific applications of photography are the composite photographs by Mr. Galton, which may be peculiarly useful in the study of anthropology. One of them is a typical family composite portrait composed of a mother and two daughters, in which all three faces are blended together. We are thus given a likeness of the female branch of the family; another, a blending of the father and mother, two sisters, and two brothers, gives the typical family group. Other pictures, in which the same principles are applied, give a typical group of engineer officers and a typical group of sappers.



O greater calamity could have befallen American science than the recent and sudden death of Professor Henry Draper. The news of it was an inexpressible shock to his friends, and was felt with painful regret by the whole community. But forty-five years of age, with the full promise of apparent health, and in the midst of an active and a brilliant career, he was cut off by an illness so short that but few had heard of it when his death was announced. In an excursion to the Rocky Mountains, in August and September, he had been subjected to severe exposure and contracted a heavy cold; he returned, however, in October, considerably recovered and able to resume his scientific labors. He gave a dinner to the National Academy of 