14 THE CHEAPEST FORM OF LIGHT. 



the amount of heat necessaiy to raise 1 gram of water, 1° Centigrade, in 

 1 minute (i. e., one small calorie) be taken as unity, then the luminous 

 radiation of the fire-fly's heat, per square cm. of exposed luminous sur- 

 face, as Ave have found, is about 0'0004 calorie in lO'"", and the total 

 luminous radiation from the most powerfully illuminating light spot of 

 the insect (the abdominal one) will not exceed O'OOOO? calorie in the 

 same time. But a small portion of this could fall upon the bolometer, 

 and that which actually reached it during the time (10'^'') required for 

 each observation, was sufficient only to affect an ordinary mercurial 

 thermometer having a bulb 1"'" in diameter by rather less than O°'0O00O23, 

 or by less than tWoto of o^^® degree centigrade. 



We have just mentioned that the total amount of heat radiation upon 

 which we have to make our investigation represents less than yo oVo u 

 calorie, while that portion of this which falls upon the apparatus would, 

 in the time of one operation, only raise the temperature of an ordinar}" 

 mercurial thermometer by less than 475-TrooT tlegree, and we have first to 

 notice the difficulty that in case invisible heat exists in company with 

 the light (and it certainl}' does exist in ordinary emanations from the 

 surface of any living creature independent of phosphorescence) we have, 

 in this minute radiation, heat of two different kinds, both invisible and 

 which it is j'et indispensable for us to discriminate. 



We are helped to do this by the consideration that while the insect, 

 like any non-luminous one, must emit " animal heat " from all its sur- 

 face, its general surface temperature is certainl}' low, since it feels cold 

 to the hand whose greater warmth excites it to shine. This heat then 

 corresponds to a temperature much below 50° Cent., and such temper- 

 atures must, as we have shown in other memoirs, be accompanied by the 

 emission of waves whose length relegates them to quite another spectral 

 region to that in which the invisible heat associated with light mainly 

 appears. We can then discriminate the rays of this invisible " animal " 

 heat without the formation of a heat spectrum by their inabilit}' to pass 

 through a glass which transmits with comparative freedom radiant heat 

 whose wave-length is less than 3'^, the latter including the region where, 

 if there be invisible heat radiated with the light, it must mainly lie. 



The heat in the spectral region of the infra-red we are considering, we 

 know in advance must be, if it bear any sort of relation to the light, 

 almost immeasurably small, and in fact it defied at first all attempts to 

 obtain not merely a quantitative measurement, but even any certain 

 experimental evidence of its existence. At last, upon July 24, with the 

 arrival of a new stock of over two dozen insects and with the aid of ex- 

 perience derived from previous failures, these heat measures were re- 

 sumed. For the first described, the thoracic light is taken. 



The insect was placed 125™ from the mirror of 25'4'='° aperture and 

 73'4'"° focus, so that its image was formed at 178°" and enlarged about 

 1.42 diameters, when a small portion of it filled an aperture equal to the 



