Sicr.XXIV. BLEACHING POWER OF SOLAR SPECTRUM. 201 



The least refrangible part of the solar spectrum pos- 

 sesses also, under certain circumstances, a bleaching 

 property, by which the metallic salts are restored to 

 their original whiteness after being blackened by ex- 

 posure to common daylight, or to the most refrangible 

 rays of the solar spectrum. 



Paper prepared with iodide of silver, when washed 

 over with ferrocyanite of potash, blackens Vapidly when 

 exposed to the solar spectrum. It begins in the violet 

 rays and extends over all the space occupied by the dark 

 chemical rays, and over the whole visible spectrum 

 down to the extreme red rays. This image is colored, 

 the red rays giving a reddish tint and the blue a bluish. 

 In a short time a bleaching process begins under the red 

 rays, and extends upward to the green, but the space 

 occupied by the extreme red is maintained perfectly dark. 

 Mr. Hunt found that a similar bleaching power is exerted 

 by the red rays on paper prepared with protocyanide of 

 potassium and gold with a wash of nitrate of silver. 



The application of a moderately strong hydriodate of 

 potash to darkened photographic paper renders it pecu- 

 liarly susceptible of "being whitened by further exposure 

 to light. If paper prepared with bromide of silver be 

 washed with ferrocyanate of potash while under the 

 influence of the solar spectrum, it is immediately dark- 

 ened throughout the part exposed to the visible rays 

 down to the end of the red, some slight interference 

 being perceptible about the region of the orange and 

 yellow. After this a bleaching action begins over the 

 part occupied by the red rays, which extends to the 

 green. By longer exposure an oval spot begins again to 

 darken about the center of the bleached space ; but if 

 the paper receive another wash of the hydriodate of 

 potash, the bleaching action extends up from the green, 

 over the region occupied by the most refrangible rays 

 and considerably beyond them, thus inducing a negative 

 action in the most refrangible part of the spectrum. 



In certain circumstances the red rays, instead of re- 

 storing darkened photographic paper to its original 

 whiteness, produce a deep red color. When Sir John 

 Herschel received the spectrum on paper somewhat 

 discolored by exposure to direct sunshine, instead of 



