Cheapest Form of Light. 273 



Owing to the motion of the insect and the varying brilliancy 

 of the light emitted, these figures (each of which is the result 

 of the mean of several trials including at least two measures) 

 still leave much to be desired. The supply of the insects, 

 which had been procured and maintained alive with difficulty, 

 however, did not allow of the experiments being further pro- 

 longed, nor of the securing a direct comparison with the solar 

 spectrum. The value of each part of the lamp spectrum 

 having, however, been independently determined with all 

 possible exactness in terms of the solar spectrum, we are 

 enabled to exhibit a comparison of the latter with the insect 

 spectrum so as to show them together (fig. 3, A and B). It 

 is assumed that the same amount of luminous intensity (i. e. 

 energy in terms of vision as determined by purely photometric 

 methods) is taken whether from the sun or the insect. The 

 subjoined curves (fig. 3, A) show the solar and the insect 

 luminosity throughout the visible spectrum on the preceding 

 assumption of the intrinsic equality, a result which, however, 

 might be liable to a slight correction of the relative places of 

 the maxima if a direct comparison with sunlight were ob- 

 tained. The important fact, however, seems to be brought 

 out almost beyond question, that when spectra are formed from 

 two equal lights, one from the sun the other from the insect, 

 the latter's spectrum terminates both at an upper and a lower 

 limit at which the solar light is still conspicuous. The con- 

 clusion follows that the insect spectrum is lacking in the rays 

 of red luminosity and presumably in the infra-red rays, 

 usually of relatively great heat ; or that it seems probable that 

 we have here light without heat, other than that heat which 

 the luminosity itself comprises and which is but another name 

 for the same enegy. 



Any other supposition would apparently involve the hypo- 

 thesis that the spectrum, which we have seen end at the red, 

 has a renewal in the invisible infra-red where the main portion 

 of the solar heat and that of all ordinary illuminants is known 

 to exist. Although this last hypothesis cannot be considered 

 to have much weight, and though we are led to agree with 

 previous observers that it may be assumed with much pro- 

 bability that the ordinary invisible heat would, if we had 

 means to observe it, be found unassociated with the fire-fly's 

 light, yet this assumption is itself far from being proof, and 

 in view of the great importance of the conclusions in question, 

 we shall now try whether it be possible to settle the point by 

 thermal measures with the bolometer. 



