434 



The Smithsonian Institution 



and again, month after month and year after year, with almost 

 interminable repetition, so that a galvanometer had, in fact, to 

 be read over a thousand times to obtain with "sufficient accu- 

 racy the position and amount of a deflection of the energy 

 curve in any single part of the invisible region. It took 

 nearly two years to fix the position of twenty lines by this 

 process, with the degree of accuracy then aimed at. 



fl 



DISTRIBUTION OF ENERGY IN 



THE SPECTRUM OF A 



60 PRISM OF SALT. 



ALLEGHENY OBSERVATORY. 



43* 



40 



33 



38 



The annexed figure shows the amount of heat in different 

 portions of the spectrum shown by the inflections of the curve 

 as obtained by this early process ; but since it took two years 

 to fix the position of twenty lines by this means, it would take 

 a hundred years to fix the position of a thousand lines, sup- 

 posing they existed; and it became evident that, if the bolom- 

 eter continued to be the only means available, new methods 

 of using it must be devised. 



Accordingly, when this work was commenced at the Smith- 

 sonian Observatory, a plan which had been under study by 

 the writer for more than ten years was introduced, by means 

 of which the work could be carried on not only with far 



