230 SOLAR SPECTRUM. [SECT xxiv. 



from the various colours that are frequently impressed on the 

 same analyzing surface, each ray having a tendency to impart 

 its own colour. Sir John Herschel obtained a coloured image 

 of the solar spectrum on paper prepared according to Mr. Tal- 

 hot's principle, from a sunbeam refracted by a glass prism and 

 then highly condensed by a lens. The photographic image 

 was rapidly formed and very intense, and, when withdrawn 

 from the spectrum and viewed in common daylight, it was 

 found to be coloured with sombre but unequivocal tints 

 imitating the prismatic colours, which varied gradually from 

 red through green and blue to a purplish black. After wash- 

 jng, the surface in water, the tints became more decided by 

 being kept a few days in the dark a phenomenon, Sir John 

 observes, of constant occurrence, whatever be the prepara- 

 tion of the paper, provided colours are produced at all. He 

 also obtained a coloured image on nitrate of silver, the part 

 under the blue rays becoming a blue brown, while that under 

 the violet had a pinkish shade, and sometimes green ap- 

 peared at the point corresponding to the least refrangible 

 blue. Mr. Hunt found on a paper prepared with fluoride of 

 silver that a yellow line was impressed on the space occupied 

 by the yellow rays, a green band on the space under the 

 green rays, an intense blue throughout the space on which 

 the blue and indigo rays fell, and under the violet rays a 

 ruddy brown appeared; these colours remained clear and 

 distinct after being kept two months. 



Notwithstanding the great variety in the scale of action of 

 the solar spectrum, the darkening or deoxydizing principle 

 that prevails in the more refrangible part rarely surpasses 

 or even attains the mean yellow ray which is the point of 

 maximum illumination ; it is generally cut off abruptly at 

 that point which seems to form a limit between the opposing 

 powers which prevail at the two ends of the spectrum. The 

 bleaching or o^xydizing effect of the red rays on blackened 

 muriate of silver discovered by M. Ritter of Jena, and the 



