112 Langley and Very — Cheapest Form of Light. 



relative importance of the light heat curve (fig. 4) only about 

 -^ of the latter can be shown in the limits of the plate. 



The curves in Plate IV 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 V supply. These curves (Plate Y) all ex- 

 hibit the spectrum on the normal scale, from that easily visi- 

 ble, lying between 0^*4 in the violet and O^ 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 considerable portion lies below 3^ 

 (and still more in that ef the candle-flame, if that were shown 

 where most of the energy would lie below 3^ or outside the 

 limits of the drawing). The curves, then, we repeat, represent 

 equal amounts of energy (which without sensible error we may 

 assume to be all exhibited as heat) and inclose equal areas. 



The total area represents in each case the expenditure of a 

 unit of cost in thermal energy, the area between G^'4 and O^'Y, 

 the proportion of this utilized as light, though as we have just 

 stated, in the case of fig. 4, the representative of the fire-fly 

 spectrum, only a fraction of this can be shown (owing to the 

 limits of the drawing.) 



Resuming then what we have said, we repeat that nature 

 produces this cheapest light at about one four-hundredth part 

 of the cost of the energy which is expended in the candle- 

 flame, and at but an insignificant fraction of the cost of the 

 electric light or the most economic light which has yet been 

 devised ; and that finally there seems to be no reason why we 

 are forbidden to hope that we may yet discover a method 

 (since such a one certainly exists and is in use on the small 

 scale) of obtaining an enormously greater result than we now 

 do from our present ordinary means for producing light. 



Appendix. 



Determination in Calories of the Heat in the Luminous {Abdom- 

 inal) Hadiation of Pyrophorus noctilucits. 



The determination is reached by two steps: (1) The calibration 

 of the galvanometer, so as to give the value of its division in 

 calories ; and (2) the inference from the observed deflection in 

 divisions of the total of calories radiated. 



1. The bolometer, whose face occupied 0-19 sq. cm. (a), gave a 

 deflection of 342 divisions (b), at a distance of 25 cm. (r) from a 

 5 cm. circular aperture filled by a blackened Leslie cube. Seen 

 from the center of this aperture, the bolometer occupied, then 



= 0-0000484 of the hemisphere, and would have received this 



