18 THE CHEAPEST FORM OP LIGHT. 



— ^ = 270400 div. The temperature of the bolometer, 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 111 cal. 

 per minute (e), which does not greatly differ from our own independent 



min. 



determinations, and for lO'""- = 0-167 (/) (the time of the galvanometer 



swing) it equals 6-185 (ef). Hence -^ = ^^^^t^ = 1462000 div. is the 



caef 0-185 



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



chI. 



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 J more than the average, which is 0-00873 (g). The measured 



dlT. 1 



radiation from this fractional part gave 0-84 div. (h) and — — 96-2 div. 



9 

 is the deflection which would be given by the total al)dominal emission, 

 or 



cal. 



96-2 X 0-000000684 = 0-0000658. 

 Since the luminous surface has an area of about I sq. cm., this corre- 



cal. 



sponds to a radiation of 0-00039 per sq. cm. of radiating surface in the 



0-0004 ""'■ 

 time of the galvanometer needle's swing, or to — . — = 0-0024 per sq. cm. 



per minute. 



(Taking the water-equivalent of the b,ulb of an ordinary mercurial 



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



0-84 X -000000684 _ o°-0000023, 

 0-25 ' 



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 tliree one-millionths 

 of a centigrade degree.) 



Note, 1901. 



These notes, added to the preceding article with the assistance of 

 Dr. H. C. Bolton, are simply intended to point out some of the princi- 

 pal additions to our knowledge of the subject which have appeared 

 during the last ten years ; and they make no pretension to be in any 

 sense a bibliography. 



About one year after publication of the preceding paper Dr. William H. 

 Seaman, of Washington, wrote an article " On the Luminous Organs of 



1 



