ON THE CHEMICAL ACTION OF THE SOLAR RADIATIONS. 



79 



beyond H to where the light entirely ceased, the dispersion was confined to 

 the immediate neighbourhood of the surface. When the solution was diluted 

 so as to be only one-tenth of the former strength, a conspicuous fixed line, 

 or rather band of sensible breadth, situated in the first group of fixed lines 

 beyond H, was observed to penetrate about an inch into the fluid. On pass- 

 ing onwards from the band above-mentioned in the direction of the more 

 refrangible rays, the distance that the incident rays penetrated into the fluid 

 rapidly decreased, and thus the rapid increase in the absorbing energy of the 

 fluid was brought into view in a part of the spectrum in which, with the 

 stronger solution, it could not be so conveniently made out, inasmuch as the 

 posterior surface of the space from which the dispersed light came almost 

 confounded itself with the anterior surface of the fluid." 



(123). The mode of operating was the same as that already described, but 

 that the experiments were made with very difl'used, and exceedingly concen- 

 trated spectra. The object of this was to determine if the less powerful rays 

 were more liable to absorption than those the energy of which had been 

 exalted by concentration. Hence the various spectra obtained varied in 

 length from 1 inch to 6 inches. 



(124). The annexed woodcut (fig.57) has been copied by the wood engraver 

 with very great care from the actual spectra obtained. [This indeed has been 

 done with all the figures of spectra in this, the quinine series.^ The space 

 from a! to a was the exact length of the visible ordinary spectrum, and under 

 the conditions of the experiment, i. e. a weak sun and a difl'used image at a 

 great distance from the slit through which light was admitted, the chemical 

 impression obtained through the trough filled with distilled water was pre- 

 cisely that represented. When the quinine solution was substituted, the 

 second image was the result. 

 (12.5). A weaker solution of the sulphate of quinine was employed, and with 



a brighter sun than the former, with a 

 less difl'used spectrum and longer ex- 

 Fig. 57. posure ; the singular elongation of the 

 image down into the orange rays, as 

 shown in fig. 58, was the result. My 

 arrangements for keeping the solar 

 image fixed being imperfect, there 

 was some motion in a horizontal direc- 

 tion, which has given an increased 

 thickness to the impressed spectrum. 



