IG THE CHEAPEST FORM OF LIGHT. 



Glass beins; interposed, the heat due to this flame radiation fell to 

 14"5 divisions, or about 8 per cent of the original radiation, showing 

 that of the quality of Bunsen fiame heat immediatel}'' 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 concentration, 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 daj^s. In some of them especially 

 luminous insect specimens were secured, which, with favorable condi- 

 tions 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 intrinsic 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 S**, while if we compare the total radiations (/. c, those 

 directly observed without the use of the glass) the contrast is still stronger. 



It follows that the insect light is accompanied by approximately 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 sub- 

 ject 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 

 spectral distribution of one unit of energy in the gas-flame spectrum 

 (plate III, Fig. 1) ; of the electric arc spectrum (plate III, Fig. 2) ; of the 

 sun (plate V, Fig. 3), and of the insect (plate III, Fig. 4). In all these 

 the abscissae are the same, the portion between 0'^-4 and O'^-T (violet to 

 red) showing the part of the energy utilized in light, while that from 

 O*^-? to 3'^ shows the part Avasted as invisible heat. The energy in each 

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

 relative imi)ortance of the light heat curve (Fig. 4) only about 2V of the 

 latter can be shown in the limits of the plate. 



The curves in plate II deal with luminous intensity only, and give no 

 means of drawing those economic conclusions which appear to follow 

 from our experiments and which the curves in plate III supply. These 

 curves (plate III) all exhibit the spectrum on the normal scale, from that 

 easily visible, l3''ing between 0'^-4 in the violet and 0'^-7 in the red, then 

 to 3'* near the limit of the glass transmission. In the case of the first 

 three, representing spectra of the gas flame, the electric arc, and the sun, 

 nearly all the energy lies above 3'^. In that of the gas flame a consider- 



