Langley and Very — Cheapest Form of Light. Ill 



deflection averaging \ of one division of the galvanometer 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 

 deflection of 177'5 divisions, showing that the total heat radia- 

 tion 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 immedi- 

 ately 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 ob- 

 tained through glass, showing the flame radiation of this quality 

 from an equal area of the same intrinsic brilliancy, i. e. invisi- 

 ble 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 favorable conditions of the galvanometer, gave 

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

 the glass to that described, it appeared that flames whose in- 

 trinsic brilliancy is nearly comparable to that of a point below 

 the middle of the candle flame, and whose total brilliancy 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^, while if we com- 

 pare the total radiations (i. e. those directly observed without 

 the use of the glass) the contrast is still stronger. 



It follows that the insect light is accompanied by approxi- 

 mately one four-hundredth part of the heat which is ordinarily 

 associated with the radiation of flames of the luminous quality 

 of those which were the subject of experiment. This value is 

 confirmed by other methods which we do not give here. It 

 will conduce to a clearer comprehension of this, if we exhibit 

 in a series of curves derived from our observations, the spec- 

 tral distribution of one unit of energy in the gas flame spec- 

 trum (Plate Y, fig. 1) ; of the electric arc spectrum (Plate Y, 

 fig. 2) ; of the sun (Plate Y, fig. 3) ; and of the insect (Plate 

 Y, fig. 4). In all these the abscissae are the same, the portion 

 between 0^4 and 0^-7 (violet to red) showing the part of the 

 energy utilized in light, while that from O*" - ? to 3*" shows the 

 part wasted as invisible heat. The energy in each case being 

 the same, the areas are the same, except that owing to the 



