Lanyley and Very — Cheapest Form of Light. 109 



luminous surface, as we have found, is about 0*0004 calorie in 

 10 sec , and the total luminous radiation from the most power- 

 fully illuminating light spot of the insect (the abdominal one) 

 will not exceed 0*00007 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 seo ) required for each 

 observation, was sufficient only to affect an ordinary mercurial 

 thermometer having a bulb l om in diameter by rather less than 

 0°*0000023, or by less than 400V00 °f one degree centigrade. 



We have just mentioned that the total amount of heat 

 radiation upon which we have to make our investigation repre- 

 sents less than ttjtWo" calorie, while that portion of this which 

 falls upon the apparatus, would in the time of one operation, 

 only raise the temperature of an ordinary mercurial thermo- 

 meter by less than y ooVoo degree, and we have first to notice 

 the difficulty that in case invisible heat exists in company with 

 the light (and it certainly does exist in ordinary emanations 

 from the surface of any living creature independent of phos- 

 phorescence), we have in this minute radiation, heat of two 

 different kinds, both invisible and which it is yet 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 surface, its general surface temperature is certainly 

 low, since it feels cold to the hand whose greater warmth ex- 

 cites it to shine. This heat then corresponds to a temperature 

 much below 50° Cent., and such temperatures 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 spec- 

 trum by their inability 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 con- 

 sidering, 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 experience 

 derived from previous failures, these heat measures were 

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



The insect was placed 125 cm from the mirror of 25*4 cm aper- 

 ture and 73*4 cm focus, so that its image was formed at 178 cm 



