210 BLEACHING ACTION. SECT. XXIV. 



served the paper from the change which it would otherwise have 

 undergone from the deoxydizing influence of the dispersed light 

 which always surrounds the solar spectrum, and this maintained 

 its whiteness. Sir John met with another instance on paper 

 prepared with bromide of silver, on which the whole of the space 

 occupied by the visible spectrum was darkened down to the very 

 extremity of the red rays, but an oxydizing action commenced 

 beyond the extreme red, which maintained the whiteness of the 

 paper to a considerable distance beyond the last traceable limit 

 of the visible rays, thus evincing decidedly the existence of 

 some chemical power over a considerable space beyond the least 

 refrangible end of the spectrum. Mr. Hunt also found that on 

 the Daguerreotype plate a powerful protecting influence is exer- 

 cised by the extreme red rays. In these cases the red and those 

 dark rays beyond them exert an action of an opposite nature to 

 that of the violet and lavender rays. 



The least refrangible part of the solar spectrum possesses also, 

 under certain circumstances, a bleaching property, by which the 

 metallic salts are restored to their original whiteness after being 

 blackened by exposure 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 rapidly 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 coloured, the red rays giving a reddish tint and the 

 blue a blueish. In a short time a bleaching process begins 

 under the red rays, and extends upwards 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 peculiarly susceptible 

 of being whitened by further exposure to light, if paper pre- 

 pared with bromide of silver be washed with ferrocyanate of 

 potash while under the influence of the solar spectrum, it is 

 immediately darkened throughout the part exposed to the visible 

 rays down to the end of the red, some slight interference being 



