264 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



The description of his instrument and mode of observa- 

 tion employed by him was published in 1843. Even 

 with this instrument, which, as we shall show, is in many 

 respects defective, Draper has succeeded in establishing 

 experimentally some of the most important relations of 

 the chemical action of light. In these experiments Dra- 

 per collected hydrogen, evolved by electrolysis over hy- 

 drochloric acid saturated with chlorine, and to this hy- 

 drogen he admitted so much chlorine, either by diffusion 

 from the saturated acid or by electrolysis, that the mixt- 

 ure consisted of nearly equal volumes of the two gases, 

 and entirely or almost entirely disappeared on exposure 

 to light. The alteration in the volume of the gaseous 

 mixture arising from the absorption of the hydrochloric 

 acid formed by the action of the light was read off on a 

 scale, and being within certain limits proportional to the 

 time of exposure, served as a measure of the chemical 

 rays.'V 



Professors Bunsen and Roscoe, having modified this 

 instrument to suit the objects they had in view, accord- 

 ingly used it in their very exhaustive and important 

 series of researches. 



The measuring lens referred to in previous paragraphs 

 is constructed upon this principle: If half the surface of 

 a convex lens be screened by an opaque body, as a piece 

 of blackened card-board, of course only half the quantity 

 of rays will pass which would have passed had the 

 screen not been interposed ; if one fourth of the lens be 

 left uncovered, only one fourth of the quantity will pass. 

 But in all these instances the focal image remains of the 

 same size as at first. Therefore by adjusting upon the 

 frame of the lens two screens, the edges of which pass 

 through its centre and are capable of rotation thereupon, 

 we shall cut off all light when the screens are applied 



