98 Langley and Very — Cheapest Form of Light. 



methods of illumination. This, it will be observed, is given as 

 a minimum value, and it is the object of the present research 

 to demonstrate that not only this possible increase but one still 

 greater is actually obtained now in certain natural processes, 

 which we know of nothing to prevent our successfully 

 imitating. 



It is now universally admitted that wherever there is light, 

 there has been expenditure of heat in the production of radia- 

 tion existing in and as the luminosity itself, since both are but 

 forms of the same energy ; but this visible radiant heat which 

 is inevitably necessary is not to be considered as waste. The 

 waste comes from the present necessity of expending a great 

 deal of heat in invisible forms before reaching even the 

 slightest visible result, while each increase of the light repre- 

 sents not only the small amount of heat directly concerned in 

 the making of the light itself, but a new indirect expenditure 

 in the production of invisible calorific rays. Our eyes recog- 

 nize heat mainly as it is conveyed in certain rapid ethereal 

 vibrations associated with high temperatures, while we have no 

 usual way of reaching these high temperatures without passing 

 through the intermediate low ones ; so that if the vocal produc- 

 tion of a short atmospheric vibration were subject to analogous 

 conditions, a high note could never be produced until we had 

 passed through the whole gamut, from discontinuous sounds 

 below the lowest bass, up successively through every lower 

 note of the scale till the desired alto was attained. 



There are certain phenomena, long investigated, yet little 

 understood, and grouped under the general name of "phos- 

 phorescent" which form an apparent exception to this rule, 

 especially where nature employs them in the living organism, 

 for it seems very difficult to believe that the light of a fire-fly, 

 for instance, is accompanied by a temperature of 2000°, or more, 

 Fahr., which is what we should have to produce to gain it by 

 our usual processes. That it is, however, not necessarily im- 

 possible, we may infer from the fact that we can by a known 

 physical process, produce a still more brilliant light without 

 sensible heat, where we are yet sure that the temperature ex- 

 ceeds this. Xo sensible heat accompanies the fire-fly's light, 

 any more than need accompany that of the Geissler tube, but 

 this might be the case in either instance, even though heat 

 were there, owing to its minute quantity, which seems to defy 

 direct investigation. It is usually assumed, with apparent 

 reason, that the insect's light is produced without the invisible 

 heat that accompanies our ordinary processes, and this view is 

 strengthened by study of the fire-fly's spectrum, which has 

 been frequently observed to diminish more rapidly toward the 

 red than that of ordinary flames. 



