262 Messrs. Langley and Very on the 



understood, and grouped under the general name of " phos- 

 phorescent " which form an apparent exception to this rule, 

 especially where nature employs them in the living organism, 

 for it seems very difficult to believe that the light of a fire-fly, 

 for instance, is accompanied by a temperature of 2000° Fahr., 

 or more, which is what we should have to produce to gain it 

 by our usual processes. That it is, however, not necessarily im- 

 possible, we may infer from the fact that we can by a known 

 physical process produce a still more brilliant light without 

 sensible heat where we are yet sure that the temperature 

 exceeds this. No sensible heat accompanies the fire-fly's light 

 any more than need accompany that of the Geissler tube ; but 

 this might be the case in either instance, even though heat 

 were there, owing to its minute quantity, which seems to defy 

 direct investigation. It is usually assumed, with apparent 

 reason, that the insect's light is produced without the invisible 

 heat that accompanies our ordinary processes \ and this view 

 is strengthened by study of the fire-fly's spectrum, which has 

 been frequently observed to diminish more rapidly toward the 

 red than that of ordinary flames. 



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 endeavoured 

 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, 

 vegetables, and organic and mineral matter. Among luminous 

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

 others of the species attain greater size, and perhaps greater 

 intrinsic brilliancy, especially the Pyropliorus noctilucus, Linn., 

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

 width 11 millim., and it has, like other 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 Smith- 

 sonian Institution, and through the kindness of Professor 

 Felipe Poey, of Havana, and Seiior Albert Bonzon, of Santiago 

 de Cuba, in the Island of Cuba, living specimens of the 

 P. noctilucus were received here during the summer of 1889. 

 I have also to acknowledge my obligations to Professor C. V. 



