ON THE CHEMICAL. ACTION OF THE SOLAR RADIATIONS. 73 



Herschel to oiFer some analogy with the reflected tints of fluor-spar and a 

 solution of sulphate of quinine, I was the more desirous of determining the 

 nature of the dispersion. It proved on examination to be nothing but false 

 dispersion, so that the appearance might be conceived to be produced by an 

 excessively fine bluish-green powder contained in a clear orange stratum, or 

 in the colourless part of the glass immediately contiguous to the coloured 

 stratum. The phenomenon has therefore no relation to the tints of fluor- 

 spar or sulphate of quinine. It is true that the very same glass which dis- 

 played a superficial reflexion of bluish-green, when examined by condensed 

 sun-light, exhibited also, in its colourless part, a little true dispersion, just as 

 another colourless glass would do. But this has plainly nothing to do with 

 the peculiar reflexion which attracts notice in such a glass." The spectrum 

 transmitted through this glass is shortened by the loss of the violet, indigo, 

 and nearly all the blue rays ; some rays are, however, still visible beyond 

 the green, which assume a reddish colour. The orange rays are extended 

 into the yellow, with which much white light is mixed, and thp least refran- 

 gible rays lose some of their illuminating power. 



(92) 68. The chemical action of the rays which Fig. 47. 



permeate this glass is confined exclusively to the ^ - 



central space of the yellow rays. On the first ex- 

 posure a mere indication of change was the only 



evidence which was obtained ; by allowing the 

 action, however, to continue for a few minutes, _ a 



taking care that the spectrum still fell upon the «* 



same space, a decided olive-brown oval spot '10 in ^ "^ 



length was obtained ; this was deepened by still pro- a/ 



longed exposure, but not enlarged. 



(93) 69. Yellow, by Carbon (?). — Beyond the green rays the blue rays 

 are still very distinct, although much reduced in their intensity of illumina- 

 tion. The least refrangible rays are not much affected by the absorptive 

 powers of this medium, although the pure brightness of the yellow is con- 

 siderably diminished. 



(94) 69. The changes which occur take place slowly, and even after a 

 prolonged exposure of many minutes, they do not arrive at 



any considerable degree of intensity. A very faint action Fig. 48. 



is observable in the space covered by the yellow rays ; this — 



action producing a weak gray patch of a similar character 

 to those already described (72). This commences at -20 

 above a\ and extends to about -50; but it here blends in so 

 gradually with the broader brown oval, that it is not pos- 

 sible to determine exactly where the former ends. A second 

 oval overlaps the first, and this one extends to the line H in 



the violet ray. There is no trace of any action beyond the 



luminous spectrum. 



(95) 72. Yellow, by /row.— This glass obliterates the „. .g 

 violet and indigo rays, but the blue space is still visible of ^^' ' 

 a reddish hue, but the light very faint ; a thin line of pure ~ 

 blue is, however, still visible immediately at the edge of the 

 green ; the least refrangible rays sufier great loss of their 

 illuminating powers. 



(96) 72. This spectrum extends from '30 above a! nearly 

 up to the end of the luminous spectrum. The colour 

 throughout is more nearly uniform than in the other exam- 

 ples. Still there is a reduction of the chemical force at -eo ; 



