110 Langley and Very — Cheapest Form of Light. 



and enlarged about 142 diameters, 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 sq mm . By 

 the preceding arrangement of the mirror an image of one of 

 the thoracic bright spots, with enough of the surrounding body 

 to represent an area of about 13 sq mm , was enlarged to nearly 

 the surface of the bolometer. 



Employing all the precautions taught by a multiplied experi- 

 ence, we obtained by a series of exposures of the bolometer to 

 the insect radiation a series of small but real galvanometer 

 deflections 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 sq mm ) 

 and about 9 sq mm 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 luminous 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 spec- 

 trum 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 Seat. 



The insects's light then is unaccompanied (in the specimen 

 subject to this experiment) by any measurable heat, but to 

 make it still more evident that this is due to the absence of 

 heat below the red (body heat not being in question) we now 

 proceed to' take an artificial flame, occupying the same area as 

 the radiating luminous part of the insect, and to see whether 

 heat is observed in it. If the flame be no brighter than the 

 insect, and the heat be nevertheless observed in it when in the 

 insect heat is lacking, it is obvious that in the latter case none 

 is observed because (sensibly) none is emitted, and this con- 

 clusion is reached a fortiori when the flame light is less than 

 that of the insect. 



July 27. Through a circular aperture 2'5 mm in diameter, there 

 was passed alternately the total radiant heat, and that trans- 

 mitted by glass from a nearly non-luminous Bunsen flame, 

 whose luminosity was very much fainter than that from the 

 insects. On this day there seemed to be an exceedingly minute 



* Described in the Memoir " On the Temperature of the Surface of the Moon," 

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



