276 Messrs. Langley and Very on the 



subject to this experiment) by any measurable heat ; but to 

 make it still more evident that this is due to the absence of 

 heat below the red (body-heat not being in question), we now 

 proceed to take an artificial flame, occupying the same area as 

 the radiating luminous part of the insect, and to see whether 

 heat is observed in it. If the flame be no brighter than the 

 insect, and the heat be nevertheless observed in it when in the 

 insect heat is lacking, it is obvious that in the latter case none 

 is observed, because (sensibly) none is emitted ; and this con- 

 clusion is reached, a fortiori, when the flame-light is less than 

 that of the insect. 



July 27. — Through a circular aperture'2'5 millim.in diameter 

 there were passed alternately the total radiant heat and that 

 transmitted by glass from a nearly non-luminous Bunsen-flame 

 whose luminosity was very much fainter than that from the 

 insects. On this day there seemed to be an exceedingly 

 minute deflexion averging \ of one division of the galvano- 

 meter-scale from the total radiation of an equal portion of the 

 abdominal light-spot of the insect, while from the flame there 

 was a mean deflexion of 177*5 divisions, showing that the total 

 heat-radiation from an equal area of a less luminous flame was 

 many hundred times that from the luminous area of the insect. 

 Glass being interposed, the heat due to this flame-radiation 

 fell to 14*5 divisions, or about 8 per cent, of the original radia- 

 tion, showing that of the quality of Bunsen-flame heat imme- 

 diately in question (that above 3^ transmissible by glass) there 

 was still something like 60 times that of the combined body 

 and luminous radiation of the insect in the far less luminous 

 flame. Subsequently, by the use of a lens giving greater con- 

 centration, measurable indications of insect-radiation above 

 3% and therefore distinct from any possible body-heat, were 

 obtained through glass, showing the flame-radiation of this 

 quality from an equal area of the same intrinsic brilliancy, 

 i. e. invisible heat and of long wave-length, but shorter than 

 3% to be about 400 times that of the insect. 



These experiments were repeated with different luminous 

 flames and with different insects on succeeding days. In some 

 of them especially luminous insect specimens were secured, 

 which, with favourable conditions of the galvanometer, gave 

 very measurable deflexions on the latter. By a similar use of 

 the glass to that described, it appeared that flames whose 

 intrinsic brilliancy is nearly comparable to that of a point 

 below the middle of the candle-flame, and whose total bril- 

 liancy is as exactly as possible comparable to that of the insect, 

 give several hundred times the heat of the latter, even if we 

 consider only that quality of heat which is found above 3% 



