Langley and Very — Cheapest Form of Light. 103 



E. DuBois. 1 — Perhaps the most important of previous 

 memoirs on phosphorescent insects is by this writer. It con- 

 tains 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 establish 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 necessary 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 process of the 

 laboratory or manufactory. With this conclusion in mind, we 

 now proceed to observations meant to demonstrate the fact 

 that this process (presumably discoverable but still unknown) 

 gives light without invisible heat. 



These observations are : 1. Photometric. 2. Thermal. 



Part 1.— Photometric Observations. 



The first impression on viewing the light of the Pyrophorus 

 noctilucus through a spectroscope is that it consists essentially 

 of a broad band in the green and yellow, while with precaution 

 we see this extending into and beyond the borders of the blue 

 and orange, but not very greatly farther, and these have been 

 taken by previous observers as its absolute limits. No one 

 appears to have experimentally and distinctly answered the 

 question, " Would the light not extend farther were it bright 

 enough to be seen ?" nor has it been proven as clearly as might 

 be desired that the result depends on the quality rather than 

 the quantity of the light, or given conclusive evidence, that if 

 the light of the insect were as bright as that of the sun it 

 would not extend equally far on either side of the spectrum. 



It is impossible to increase the intrinsic brilliancy by any 

 optical device, but if it be impossible to make the light of the 

 insect as bright as that of the sun, it is on the other hand quite 

 possible to make the light of the sun no brighter than that of 

 the insect, and this would appear to be the first step in obtain- 

 ing a definite proof that the apparently narrow limits of the 

 insect's spectrum are due to the intrinsic quality of the light 



1 " Bulletin de la Societe Zoologique de France," parts 1, 2 and 3, 18S6. 



