" N" RAYS IN SOLAR RADIATION 27 



small flame is by far the most convenient and 

 precise of all processes for determining the 

 position of the foci. Operating with the small 

 spark is much harder, because the spark is 

 rarely very regular. 



I feel bound to reproduce, textually, here a 

 passage in a letter which M. Gustave le Bon 

 has done me the honour of writing 



" M, Gustave le Bon had indicated, as far back as 

 seven years ago, that flames emit, independently of 

 the radio-active emanations since observed by him, 

 radiations of large wave-length, capable of traversing 

 metals, and to which he had given the name of black 

 light ; but while assigning these a place intermediate 

 between light and electricity, he had not exactly 

 measured their wave-length, and the method he 

 had employed to reveal their presence was very un- 

 certain." 



The method referred to was the photo- 

 graphic method. Personally, I have not been 

 able to obtain any photographic effect of the 

 rays I have studied (see p. 16). 



