76 Ogden Nicholas Rood. 



and he recurred to it again and again. He was ultimately 

 fortunate in discovering, in 1893, a novel and original photo- 

 metric method which is independent of color, and which 

 depends upon the shock which the retina receives when one 

 illuminated surface is quickly withdrawn and replaced by an- 

 other of different brightness. The flickering effect thus pro- 

 duced disappears when the two lights are of equal intensity. 

 Tin's idea he further elaborated later, and in 1899, lie pub- 

 lished in this Journal an article upon the Flicker Photometer, 

 with a more complete description, and showed by elaborate 

 tests and verifications that the accuracy attainable with this 

 instrument, in comparing differently colored lights, is about the 

 same as for white, or like colors, in the ordinary photometer. 

 He applied it also with great effect in the study of color- 

 blindness, and brought out, with striking emphasis, the inter- 

 esting fact that eyes supposed to be normal differ greatly in 

 their perception of colors, and that it is rare to find two per- 

 sons who agree in their color vision. 



Another subject which received much attention from him 

 was the study of the character of various complex colors, 

 especially those of the different pigments, and he devised a 

 system by which it was possible to express the composition of 

 such colors by the proportion of certain standard colors in their 

 make up, thus for the first time introducing definite quantita- 

 tive methods in the study of color and color-contrasts. 



The results of Professor Pood's studies of color for many 

 years especially fitted him to write a treatise upon the subject, 

 and in 1879 he published a volume entitled " Modern Chro- 

 matics, with Applications to Art and Industry" (New York, 

 D. Appleton & Co.), it being one of the volumes of the Inter- 

 national Scientific Series. To the title, in a later issue, were 

 added the words, "Student's Text-Book of Color." This addi- 

 tion suggests the purpose and scope of the work, which was 

 intended for the inexperienced student interested in the prac- 

 tical uses of color, as well as for those familiar with the prin- 

 ciples of optical science. It is a happy example of clear and 

 simple exposition, that is at the same time of a strictly scien- 

 tific character. The book is full of interesting suggestions, 

 and the results were carefully verified by innumerable novel 

 and ingenious experiments which were original. It forms an 

 indispensable hand-book for the artist, as well as for those 

 interested in the practical applications of color. 



It may be added that Professor Pood's work upon this book 

 was greatly facilitated by his own experience as an artist. As 

 early as his residence in Munich he had practiced painting in 

 oil, and attained a high degree of proficiency. He had great 

 skill in drawing, and became expert in painting in water-colors, 



