Cheapest Form of Light. 267 



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 that 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 un- 

 known) gives light without invisible heat. 



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



Part I. — 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 pre- 

 caution 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 evi- 

 dence, 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 

 obtaining a definite proof that the apparently narrow limits 

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

 light and not to its feeble intensity. The only conclusive 

 method of determining this would appear to be to balance the 

 light from the insect with that of a definite portion of sunlight 

 by any ordinary photometric device ; and having taken this 

 sunlight as nearly equal as possible to that of the insect, 

 though certainly not greater, to let this determined quantity 

 fall on the slit of a spectroscope at the same time with the 

 light from the insect, two spectra being formed one over the 

 other in the same field and at the same time. 



The actual doing this is not so easy as it might appear, 

 owing to experimental difficulties connected with the insect, 



