PRESIDENTIAL ADDRESS, B.A. 1871. 



month of October 1850. A prism and lenses were 

 arranged to throw upon a screen an approximately- 

 pure spectrum of a vertical electric arc between 

 charcoal poles of a powerful battery, the lower one 

 of which was hollowed like a cup. When pieces of 

 copper and pieces of zinc were separately thrown 

 into the cup, the spectrum exhibited, in perfectly 

 definite positions, magnificent well-marked bands 

 of different colours characteristic of the two metals. 

 When a piece of brass, compounded of copper and 

 zinc, was put into the cup, the spectrum showed all 

 the bands, each precisely in the place in which it 

 had been seen when one metal or the other had 

 been used separately. 



It is much to be regretted that this great 

 generalisation was not published to the world 

 twenty years ago. I say this, not because it is to 

 be regretted that Angstrom should have the credit 

 of having in 1853 published independently the 

 statement that " an incandescent gas emits lumi- 

 " nous rays of the same refrangibility as those 

 " which it can absorb " ; or that Balfour Stewart 

 should have been unassisted by it when, coming to 



