216 PROCEEDINGS OF THE AMERICAN ACADEMY 



XVII. 



THEORY OF ABSORPTION-BANDS IN THE SPECTRUM, 



AND ITS BEARING IN PHOTOGRAPHY 



AND CHEMISTRY. 



By Dr. Rodert Amort. 



Presented Jan. 9, 1878. 



In order to present this communication clearly, I must apologize for re- 

 mmding you of certain facts, which probably are familiar to you as well 

 as to myself; and these may be summarized in the following review. 

 The rays of light absorbed by a colored solution cannot always be de- 

 termined by its apparent color ; for instance, an aniline alkali blue salt 

 of commerce will extinguish or neutralize green and yellDW rays (be- 

 tween b and D lines in the solar spectrum), eosin. (fluorescein) of com- 

 merce absorbs only the green rays, its greatest intensity being at the 

 E lines, it should also be observed that this latter substance by trans- 

 mitted light appears red, and by reflected light fluorescent green. 



By the term " absorption " it is ordinarily meant tliat a colored solution 

 by transmitted light allows only those light rays to pass which do not 

 belong to itself: in other words, certain of these rays are extinguished 

 or neutralized, whilst others are transmitted. To determine correctly 

 which of these rays are absorbed and which are transmitted, we must 

 examine, by means of a glass or other transparent prism, the sunlight 

 (or other incandescent light) transmitted through the colored solution. 



The explanation of absorption is usually given somewhat in the 

 following manner : — 



A colored solution owes its absorption-bands, seen in the spectrum 

 from its transmitted sunlight, to the fact that the ether molecules are 

 excited by the sunlight, or other source of illumination, to move in 

 undulatory vibrations ; and that certain of these waves are of the same 

 length with those of the solution, and hence are extinguished or with- 

 held, whilst all others of unequal length are allowed to pass through. 

 This explanation is founded upon the analogy offered by the solar or 

 Frauenhofer lines, which appear black because their monochromatic 



