8 THE CHEAPEST FORM OF LIGHT. 



Rohin and Lahoulbene'^ find the luminous organs of P. noctilucus com- 

 posed of irregularly pol^iiedral cells, 0-04'"" to O-Oe"""" thick, between 

 which pass very numerous fine trachese and nerves. The inner face of 

 the organ is composed of adipose tissue, and the outer of a transparent 

 modification of the ordinary chitinous covering of the insect. The 

 autliors conclude that the light is due to chemical decomposition of a 

 nitrogenous body with formation of cr3^stalline urates. 



Jousset de Bellesme f finds that although the phosphorescent cells, when 

 separated from the body of the insect, continue to glow for several hours, 

 yet if crushed the}^ instantly lose their illuminating power, which indi- 

 cates that for the production of the light the living cells must retain 

 their integrit}^ and that they are not mere reservoirs of a phosphorescent 

 substance, but continuous generators of it. He surmises that the light- 

 giving substance may be phosphureted hydrogen. 



MeldolaX is quoted by Si)iller§ as having examined the glow-worm 

 spectrum and determined its approximate limits. 



Conroy \\ finds the glow-worm's light green, and in a small direct 

 vision spectroscope showing a continuous spectrum from C to 6, appear- 

 ing like a broad band of green light extending from O'^'olS to O'^oST 

 with a faint continuous spectrum into the red to 0'^'656. 



R. Du Bois.^ — Perhaps the most important of previous memoirs on 

 phosphorescent insects is by this writer. It contains an account of 

 photometric measures in wave-length scale, and also of heat measures 

 with the thermopile. The latter represent the only attempt even, in 

 this direction, I know of, and seem to be judiciously made but to be 

 insufficient (on account of the limitations of such apparatus) to estab- 

 lish the author's conclusion that the light is accompanied by no sensi- 

 ble heat. This conclusion, we repeat, though very probably correct, 

 does not seem to rest on the evidence of an apparatus of at all the neces- 

 sary sensitiveness. This memoir, however, appears to be in general an 

 excellent one, and well worthy the student's attention. 



From all these statements it is abundantly clear that not only physicists 

 and chemists, but naturalists, have been led to conclude that this light 

 is not associated indissolubly with any so-called vital principle or vital 

 process, but it is a result of certain chemical combinations, and that 

 nothing forbids us to suppose it may be one day produced by some i)ro- 

 cess of the laboratory or manufactory. With this conclusion in mind, 

 we now proceed to observations meant to demonstrate the fact that this 



*C. R.,lxxvii, p. 511, 1873. 



tC. R., xc, p. 318, 1880. 



X " Proc. Entomological Soc," p. iii, 1880. 



I "Nature," vol. xxvi, p. 343. 



II "Nature," vol. xxvi, p. 319, 1882. 



1[ " Bulletin de la Societe Zoologique de France," parts 1, 2, and 3, 1886. 



