Langley and Very — Cheapest Form of Light. 99 



Nevertheless, this though a highly probable and reasonable 

 assumption, remains assumption rather than proof ; until we 

 can measure with a sufficiently delicate apparatus the heat 

 which accompanies the light and learn not only its quantity, 

 but what is more important, its quality. Apart from the 

 scientific interest of such a demonstration, is its economic value, 

 which may be inferred from what has already been said. I 

 have therefore thought it desirable to make the light of the 

 fire-fly the subject of a new research, in which it is endeavored 

 to make the bolometer supplement the very incomplete evi- 

 dence obtainable from the visible spectrum. 



As we may learn from elementary treatises, phenomena of 

 phosphorescence are common to insects, fishes, mollusks, vege- 

 tables, and organic and mineral matter. Among luminous 

 insects the fire-fly of our fields is a familiar example, though 

 other of the species attain greater size, and perhaps greater 

 intrinsic brilliancy, especially the Pyrophorus noctilucus Linn., 

 found in Cuba and elsewhere. Its length is about 37 mm , width 

 ll mm , and it has, like Pyrophori, three light reservoirs. — two in 

 the thorax and one in the abdomen. To procure this Cuban 

 fire-fly I invoked the aid of the Smithsonian Institution, and 

 through the kindness of Professor Felipe Poey, of Havana, and 

 Senor Albert Bonzon, of Santiago de Cuba, in the Island of 

 Cuba, living specimens of the Pyrophorus noctilucus were 

 received here during the summer of 1889. I have also to 

 acknowledge my obligations to Professor C. Y. Pi ley and to 

 Professor L. A. Howard, to whose knowledge and kind care I 

 am doubly indebted. 



After a preliminary spectral examination in Washington, I 

 found it more convenient to continue the research at the Alle- 

 gheny Observatory by means of the very special apparatus sup- 

 plied by the liberality of the late William Thaw of Pittsburgh, 

 for researches in the lunar heat-spectrum.* Photometric 

 measurements throughout the spectrum of the insect's light 

 were also made. 



I have indicated the steps of the investigation, but the ex- 

 periments have been so largely and so intelligently made by 

 Mr. F. W. Very, that it is just to consider him as an associate 

 rather than an assistant in the researches. I shall accordingly 

 in what follows not discriminate between what each has con- 

 tributed. 



Historical Notes. 



We make no attempt to give any bibliography of the sub- 

 ject, and these notes are confined to what seems important in 

 the history of the physical side of it. 



* Described in the Memoirs of the National Academy, vol. iv, Part II, p. 112. 



