4 THE CHEAPEST FORM OF LIGHT. 



considered as waste. The waste comes from the present necessity of ex- 

 pending a great deal of heat in invisilde forms before reaching even the 

 slightest visible result, while each increase of the light represents 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 recognize heat mainly as it is conveyed in cer- 

 tain rapid ethereal vibrations associated with high tem[)eratures, while 

 we have no usual way of reaching these high temperatures without pass- 

 ing through the intermediate low ones, so that if the vocal production of 

 a short atmospheric vibration were subject to analogous conditions, a 

 high note could never be produced until we had i>assed through the 

 whole gamut, from dicontinuous sounds below the lowest bass up suc- 

 cessively through ever}'' lower note of the scale till the desired alto was 

 attained. 



Tiiere are certain phenomena, long investigated, yet little understood, 

 and grouped under the general name of " phosphorescent " 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 2,000°, or 

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

 usual processes. That it is, however, not necessarily impossible, 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 exceeds this. No sensible heat accompanies the 

 fire-fly's light, any more than need accom})any 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 inves- 

 tigation. It is usually assumed, with apparent reason, that the insect's 

 light is produced without the invisible heat that accompanies our or- 

 dinary 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. 



Nevertheless, this, though a highly probable and reasonable assump- 

 tion, remains assumption rather than proof, until we can measure with 

 a sufficienth' delicate apparatus the heat which accompanies the light 

 and learn not only its quantity, but, what is more important, its quality. 

 Apart from the scientific interest of such a demonstration is its economic 

 value, which may be inferred from what has already been said. I have 

 therefore thought it desirable to make the light of tiie fire-fly the subject 

 of a new research, in which it is endeavored to make the bolometer sup- 

 plement the very incomplete evidence obtainable from the visible spec- 

 trum. 



As we may learn from elementary treatises, phenomena of phosphor- 

 escence are common to insects, fishes, moUusks, vegetables, and organic 

 and mineral matter. Among luminous insects the fire-fly of our fields 



