232 BLEACHING POWER OF SOLAR SPECTRUM. [SECT. xxiv. 



plate a powerful protecting influence is exercised 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 white- 

 ness 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 ferrocyanke 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 per- 

 fectly 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 sus- 

 ceptible of being whitened by further exposure to light. If 

 paper prepared with bromide of silver be washed with fer- 

 rocyanate 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 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 centre 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 



