260 Messrs. Langley and Very on the 



of an accurate number being obtained in this way. The 

 figure obtained is about 51 x 10 G , a value which agrees with 

 the observations better than that found by experiment. 



I should add that I do not attach any importance to the 

 discrepancies referred to. The experiments were made 

 mainly with the view of showing the oscillating spark to an 

 audience more clearly than is possible by the method of Dr. 

 Lodge. For the purpose of measurement a window-glass 

 condenser is not suitable at all, and I do not think the rotating 

 lenses are so suitable as the method employed by Dr. Lodge, 

 in which the plate itself is made to rotate in its own plane, 

 for then the centre can be determined with the greatest 

 accuracy. 



The actual gain in number of sparks produced by the trap 

 was found by making three experiments, in which all the 

 conditions were the same except those specified. 



A. No trap . . -j 1 ^ inch spark 32 elementary sparks. 



B. No trap . . ^o mcn spark 29 „ 



C. Trap giving 1 



potential > 20 mcn spark 37 „ 



of A. j 



I should in conclusion state that the measurements given 

 at the end of these notes were made after the meeting of the 

 Physical Society at which the paper was read, and that some 

 other additions have been made to the fourth part of these 

 notes. 



XXXI. On the Cheapest Form of Light, from studies at the 

 Allegheny Observatory. By S. P. Langley and F. W. 

 Very*. 



THE object of this memoir is to show, by the study of the 

 radiation of the fire-fly, that it is possible to produce 

 light without heat other than that in the light itself ; that 

 this is actually effected now by nature's processes ; and that 

 these are cheaper than our industrial ones in a degree hitherto 

 unrealized. By " cheapest " is here meant the most econo- 

 mical in energy, which for our purpose is nearly synonymous 

 with heat ; but as a given amount of heat is producible by a 

 a known expenditure of fuel at a known cost, the word 



* Communicated by the Authors, to whom we are likewise indebted 

 for the cliches. 



