THE CHEAPEST FORM OF LIGHT. 17 



able portion lies below '>(and still more in tbat of the candle flame, if 

 that were shown where most of the energy would lie below <> 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 betAveen 0'^-4 and 0''-7 the proportion of 

 this utilized as liyht^ 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 chea])est light at about one-four-hundredth part of the cost of the 

 energy which is ex})ended in the candle flame, and at but an insignifi- 

 cant 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 (Abdominal) Radia- 

 tion of Pi/rophorus noctilucus. 



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 O'lO sq. cm. (a), gave a deflection 

 of 342 divisions (/>) at a distance of 25 cm. (r) from a 5 cm. circular aper- 

 ture filled by a blackened T^eslie cube. Seen from the center of this 



aperture, the bolometer occupied, then y^ — 0-0000484 of the hemi- 

 sphere, and would have received this fraction of the total radiation, ex- 

 cept that being placed exactly oj)i)osite the radiating surface, more than 

 the mean radiation fell on it in a proportion which calculation shows to 

 be a))Out h The fraction of the total radiation which it actually received, 

 then, was 0-0000645 {c). 



Accordingly the total radiation would have caused a deflection 



A = 5300000 divisions. 

 c 



The surface of the cube was at a temperature of 99° Cent., and was 

 limited by the diaphragm to an area of 196 sq. cm. (d.). The total ra- 

 diation from one centimeter, then, would have caused a deflection of 



