108 Langley and Very — Cheapest Form of Light. 



spectrum. The value of each part of the lamp spectrum hav- 

 ing, 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 (Plate IV, figs. 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 (Plate IY, fig. 1) 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 conclu- 

 sion 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 lumin- 

 osity itself comprises and which is but another name for the 

 same energy. 



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 prob- 

 ability 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. 



Part 2. Thermal Observations. 



To give an idea of the amount of heat at our disposition for 

 experiment, and of the actual minuteness of the radiation 

 which proceeds from even the most luminous tropical insect, 

 we may say that if that rate of radiation from a lamp-black 

 surface 1 sq. cm. in area, which represents the amount of heat 

 necessary to raise 1 gram of water, 1° centigrade, in 1 minute 

 (i. e. one small calorie) be taken as unity, then the luminous 

 radiation of the fire fly's heat, per square cm. of exposed 



