but the visible brightness 525 times as large as that of the terrestrial 

 source of light. 



In the last section of this communication the chemical action of 

 the constituent parts of the solar spectrum is investigated. The 

 sun's rays were reflected from a Silbermann's heliostat, and after 

 passing through a narrow slit, they were decomposed by two quartz 

 prisms. The spectrum thus produced was allowed to fall upon a 

 white screen covered with a solution of quinine, and any desired por- 

 tion of the rays could be measured by a finely-divided scale, and the 

 position noted by observation of the distances from the fixed lines. 

 For the purpose of identifying the fixed lines in the lavender rays, 

 the authors were, by the kindness of Mr. Stokes, allowed the use of 

 an unpublished map of the most refrangible portion of the spectrum, 

 prepared by that gentleman. As the various components of white 

 light are unequally absorbed by the atmosphere, it was obviously 

 necessary to conduct all the measurements so quickly after one 

 another, that no appreciable difference in the thickness of the column 

 of air passed through should occur. 



This has been accomplished, and a series of exact measurements 

 of the chemical actions of the spectrum for one particular zenith- 

 distance of the sun obtained. The action on the sensitive gas shows 

 the existence of several maxima of chemical intensity in the spectrum. 

 Between the lines G in the indigo and H in the violet the greatest 

 action was observed, whilst another maximum was found to lie near 

 the line I in the ultra-violet rays. Towards the red or least refran- 

 gible end of the spectrum, the action became imperceptible about the 

 line D in the orange, but at the other end of the spectrum the action 

 was found to extend as far as Stokes' s line U, or to a distance from 

 the line H greater than the total length of the ordinary visible spec- 

 trum. Tables and curves representing the action are given. 



VOL. x. 



