THE CHEAPEST FORM OF LIGHT. 15 



bolometer emplo3'ecl, which was selected from the most sensitive of those 

 used in previous researches in lunar heat and had an aperture of 19"i™™. 

 By the preceding arrangement of the mirror an image of one of the tho- 

 racic bright spots, with enough of the surrounding bod)^ to rej)resent an 

 area of about 13"' """, was enlarged to nearly the surface of the bolometer. 

 ICmploying all the precautions taught by a multiplied experience, 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 radiations come jointly from the luminous spot (area 3 to 4^'i """) 

 and about 9''' '"'° of the surrounding liody. 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 jnissing 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 cai)able of affecting the apparatus, though this 

 was so sensitive as to promptly respond to the feeble bod}' 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 subject to 

 this experiment) by any measurable heat, but to make it still more evi- 

 dent that this is due to the absence of heat below the red (bod}' 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 

 Avhether 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 be- 

 cause (sensibly) none is emitted, and this conclusion is reached a fortiori 

 when the flame light is less than that of the insect. 



July 27. Through a circular aperture 2'5""" in diameter there was 

 passed alternately tlie total radiant heat, and that transmitted 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 deflection averaging i of one division of the 

 galvanometer scale from the total radiation of an equal portion of the 

 abdominal light spot of the insect, while from the flame there was a 

 mean deflection of 177'5 divisions, showing that the total heat radiation 

 from an equal area of a less luminous flame was many hundred times 

 that from the luminous area of the insect. 



* Described in the Memoh- " On the Temi)erature of the Surface of the Moon," 

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



