MEMOIR XVIII.] Till. < HI.oK IIVDKoGEN PHOTOMETER. 



251 



between the numbers do not greatly exceed the possible 

 errors of observation. It may be remarked that the 

 third aud fourth experiments were made with a different 

 lamp. 



TABLE I. 



Showing that when the radiant source is constant, the amount of move- 

 ment in the photometer is directly proportional to the times of exposure. 



Though a certain amount of radiant heat from a source 



O 



so highly incandescent as that here used will pass the 

 lens, its effects can never be mistaken for those of the 

 chemical rays. This is easily understood when we re- 

 member that the effect of such transmitted heat would 

 be to expand the gaseous mixture, but the chemical ef- 

 fect is to contract it. 



Next, the indications of the photometer are strictly 

 proportional to the quantity of rays that have impinged 

 upon it ; a double quantity producing a double effect, a 

 triple quantity a threefold effect, etc. 



A slight modification in the arrangement (Fig. 40) 

 enables us to prove this in a satisfactory way. The lens, 

 D, being mounted in a square wooden frame, can easily 

 be converted into an instrument for delivering at its 

 focal point, where the sentient tube is placed, measured 

 quantities of the chemical rays, and thus becomes an 

 invaluable auxiliary in those researches which require 

 known and predetermined quantities of radiations to be 



