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THE CHLOK-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



measured out. The method of doing this will be de- 

 scribed in a subsequent part of this Memoir. 



In order, therefore, to prove that the indications of 

 the photometer are proportional to the quantity of im- 

 pinging rays, place this measuring lens in the position 

 D, setting its screens at an angle of 90. Remove the 

 screen E, and determine the effect on the photometer for 

 one minute. At the close of the minute, and without 

 loss of time, turn one of the screens so as to give an 

 angle of 180, and now the effect will be found double 

 what it was before, as in the following table : 



TABLE II. 



Showing that the indications of the photometer are proportional to the 

 quantity of incident rays. 



I have stated in the commencement of this paper that 

 the action upon the photometer is limited to a ray which 

 corresponds in refrangibility to the indigo, or, rather, that 

 in the indigo space its maximum action is found. The 

 table on the following page serves at once to prove 

 this fact, and also to illustrate the chemical force of the 

 different regions of the spectrum. 



In this table the spaces are equal ; the centre of the 

 red, as insulated by cobalt blue glass, is marked as 

 unity ; the centre of the yellow, insulated by the same, 

 being marked 3 ; the intervening region being divided 

 into two equal spaces, and divisions of the same value 

 carried on to each end of the spectrum. 



As instruments will no doubt be hereafter invented 

 for measuring the phenomena of different classes of rays, 



