SBCT. XXIV. THE SOLAR SPECTRUM. 199 



in most instances where there fa least light and heat, 

 and even in the space where both sensibly cease. 



Not only the intensity but the kind of action is differ- 

 ent in the different points of the solar spectrum, as 

 evidently appears from the various colors that are fre- 

 quently impressed on the same analyzing surface, each 

 ray having a tendency to impart its own color. Sir John 

 Herschel obtained a colored image of the solar spectrum 

 on paper prepared according to Mr. Talbot'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 with- 

 drawn from the spectrum and viewed in common day- 

 light it was found to be colored with sombre but une- 

 quivocal tints imitating the prismatic colors, which varied 

 gradually from red through green and blue^to a purplish 

 black. After washing 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 preparation of the paper, 

 provided colors are produced at all. He also obtained a 

 colored 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 colors 

 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 deoxy- 

 dizing 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 ox- 

 ydizing effect of the red rays on blacke'ned muriate of 

 silver discovered by M. Ritter of Jena, and the resfora- 



