SKETCH OF PROFESSOR HENRY DRAPER. 405 



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. War- 

 nerke 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 re- 

 membered 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 dai'kening 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 scien- 

 tific applications of photography are the composite photographs by 

 Mr. Galton, which may be peculiarly useful in the study of anthro- 

 pology. 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. 



SKETCH OF PROFESSOR HENRY DRAPER. 



~T^T~0 greater calamity could have befallen American science than 

 -L.N 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 sub- 

 jected 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 Sci- 



