THE CHEAPEST FORM OF LIGHT. 13 



Owing to the motion of the insect and the varying brillianc}' 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 main- 

 tained alive with difficulty, however, did not allow of the experiments 

 being further prolonged, nor of the securing a direct comparison with 

 the solar 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 11, 

 Figs. A and B). It is assumed that the same amount of luminous in- 

 tensity (i. e., energy in terms of vision as determined l)y purely pho- 

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

 subjoined curves (i)late II) sliow the solar and the insect luminosity 

 throughout the visible spectrum on the preceding assum})tion of the in- 

 trinsic equality, a result whicli, however, might be liable to a slight cor- 

 rection of the relative places of the maxima if a direct comparison with 

 sunlight were obtained. The imi)ortant fact, however, seems to be 

 brought out almost beyond question that when spectra are formed from 

 two equal lights, one from tliesun, 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 conclusion follows that the insect 

 spectrum is lacking in the rays of red luminosity and ])resumably in 

 the infra-red rays, usually of relatively great heat, or that it seems prob- 

 al)le that we have here light without heat, other than that heat whicli the 

 luminosity itself comprises and which is but another name for the same 

 energy. 



An}^ other sui)position would apparently involve the h3'-pothesis 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 hypoth- 

 esis cannot be considered to have much w,eight, and though we are led 

 to agree with previous observers that it may be assumed with mucli 

 probability 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 im- 

 portance of the conclusions in question^ we shall now tr}'- whether it be 

 possible to settle the jjoint by thermal measures with the bolometer. 



Part 2. — Thermal Observations. 



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

 ment, 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 radi- 

 ation from a lamp-black surface 1 square cm. in area, which represents 



