Cheapest Form of Light. 275 



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 centim. from the mirror of 

 25*4 centim. aperture and 73*4 centim. focus, so that its 

 image was formed at 178 centim. and enlarged about 1*42 

 diameter, when a small portion of it filled an aperture equal 

 to the bolometer employed, which was selected from the most 

 sensitive of those used in previous researches in lunar heat, 

 and had an aperture of 19 square millim. By the preceding 

 arrangement of the mirror an image of one of the thoracic 

 bright spots, with enough of the surrounding body to repre- 

 sent an area of about 13 square millim., was enlarged to nearly 

 the surface of the bolometer. 



Employing all the precautions taught by a multiplied expe- 

 rience, we obtained by a series of exposures of the bolometer 

 to the insect-radiation a series of small but real galvanometer- 

 deflexions which represent the excess of total heat-radiations 

 from the insect over those from a metal plate of a temperature 

 of about 25° C. forming the background. These heat-radia- 

 tions come jointly from the luminous spot (area 3 to 4 square 

 millim.), and about 9 square millim. of the surrounding body. 

 To determine their characters, we interposed a sheet of glass * 

 which cut off all the observed heat. The heat from the lumi- 

 nous spectrum and from a spectral region below it extending 

 to about 3^ (30,000 tenth-meters) was known to be capable of 

 passing through this glass. The evidence, then, is that there 

 is no heat in the spectrum below this feeble radiation from 

 the luminous thoracic region sufficient to be capable of 

 affecting the apparatus, though this was so sensitive as to 

 promptly respond to the feeble body-radiation from the 

 somewhat larger section of the luminous and non-luminous 

 surface. 



Continuation on Abdominal Heat. 



The insect's light, then, is unaccompanied (in the specimen 



* Described in the memoir " On the Temperature of the Surface of the 

 Moon," Mem. Nat. Acad, of Sciences, vol. iii., as "B." 



