Langley and Very — Cheapest Form of Light. 113 



fraction of the total radiation, except that being placed exactly 

 opposite the radiating surface, more than the mean radiation fell 

 on it in a proportion which calculation shows to be about \. 

 The fraction of the total radiation which it actually received, 

 then was 0-0000645 (c). 



Accordingly the total radiation would have caused a deflection 



b 



- = 5300000 divisions. 

 c 



The surface of the cube was at a temperature of 99° Cent, and 



was limited by the diaphragm to an area of 19*6 sq. cm. (d). 



The total radiation from one centimeter then would have caused 



a deflection of —=- = 270400 div. The temperature of the bolo- 

 meter, which was that of the apartment was 20° C. According 

 to Dulong and Petit's law, the radiation from such a surface at 

 99° C. to one at 20° C. would be I'll cal. per minute (e), which 

 does not greatly differ from our own independent determinations, 



min. 



and for 10 sec, =0-167 (/"), (the time of the galvanometer swing), 



cal ' / « TT b 270400 t ,.V', 



it equals 0-185 (ef). Hence -^-„= = 1462000 div. is the 



H KJ J cdef 0-185 



potentiality of work in 1 calorie, to be expressed in the swing of 



cal. 



the galvanometer needle, and 1 div. =0-000000684. 



2. The galvanometer received the fire-fly radiation through a 

 lens which occupied 0*00655 of a hemisphere, and would have 

 transmitted this fraction of the total heat, except for its position, 

 which caused it to transmit ^ more than the average, which is 

 0-00873 (g). The measured radiation from this fractional part 



div. Ji 



gave 0*84 div. (h) and - = 96'2 div. is the deflection which would 

 be given by the total abdominal emission, or 



cal. 



96-2x0-000000684 = 0-0000658. 

 Since the luminous surface has an area of about ^ sq. cm., this 



cal. 



corresponds to a radiation of 0-00039 per sq. cm. of radiating sur- 



■ ! • Pi, -., , 0-0004 



face in the time of the galvanometer-needle s swing, or to — — 



cal. J 



= 0-0024 per sq. cm. per minute. 



(Taking the water-equivalent of the bulb of an ordinary mer- 



curial thermometer 1 cm. in diameter at 0-25 we find 



0-84 X -000000684 



0-25 



= 0°-0000023 



showing that if such a thermometer were placed in the position 

 occupied by the bolometer its rise during the time of the latter's 

 exposure to the radiation of the insect would be between two and 

 three one-inillionths of a centigrade degree.) 

 Am. Jour. Sci.— Third Series, Vol. XL, No. 236. — August, 1890. 



