SECT. XXIV. CHEMICAL SPECTRUM. 209 



occurrence, whatever be the preparation 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 appeared 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 

 oxydizing effect of the red rays on blackened muriate of silver 

 discovered by M. Eitter of Jena, and the restoration by the 

 same rays of discoloured gum guaiacum to its original tint by Dr. 

 Wollaston, have already been mentioned as giving the first indica- 

 tions of that difference in the mode of action of the chemical rays at 

 the two ends of the visible spectrum, now placed beyond a doubt. 



The action exerted by the less refrangible rays beyond and 

 at the red extremity of the solar spectrum, in most instances, 

 so far from blackening metallic salts, protects them from the 

 action of the diffused daylight : but, if the prepared surface has 

 already been blackened by exposure to the sun, they possess the 

 remarkable property of bleaching it in some cases, and under 

 other circumstances of changing the black surface into a fiery red. 



Sir John Herschel, to whom we owe most of our knowledge 

 of the properties of the chemical spectrum, prepared a sheet of 

 paper by washing it with muriate of ammonia, and then with 

 two coats of nitrate of silver ; on this surface he obtained an 

 impression of the solar spectrum exhibiting a range of colours 

 very nearly corresponding with its natural hues. But a very 

 remarkable phenomenon occurred at the end of least refrangi- 

 bility ; the red rays exerted a protecting influence which pre- 



