CHANGES PRODUCED BY ACTINISM. 175 



The picture which we receive to-day, unless we adopt 

 some method of securing its permanency, fades away 

 before the morrow, and we try to restore it in vain. 

 "With some of our chemical preparations this is very 

 remarkably shown, but by none in so striking a manner 

 as by paper prepared with the iodide of platinum, which, 

 being impressed with an image by heliographic power, 

 which is represented by dark brown tints, restores itself 

 in the dark, in a few minutes, to its former state of a 

 yellow colour, and recovers its sensibility to sunshine.* 

 The inference we alone can draw from all the evidences 

 which the study of actino-chemistry affords, is, that the 

 hours of darkness are as necessary to the inorganic 

 creation as we know night and sleep to be to the organic 

 kingdom. But we must not forget that there does exist 

 in the solar rays a balance of forces which materially 

 modifies the amount of disturbing influence exerted by 

 them on matter. Not only do we find that the chemical 

 action is not extended over the whole length of the 

 prismatic spectrum, but we discover that over spaces, 

 which correspond with the maximum points of light and 

 heat, a protective action is exerted. That is, that 

 highly sensitive photographic agents, which blacken 

 rapidly under exposure to diffused daylight, are entirely 

 protected from change in full sunshine, if at the same 

 time as a strong light is thrown upon them by reflection, 

 the yellow and extra red rays are brought to bear upon 

 their surface. Not only so, but by employing media 

 which will cut off all the chemical rays of the spectrum, 

 admitting freely at the same time the luminous and 

 calorific rays, we find that a protected band, the length. 



of the Solar Spectrum ; by Sir John F. W. Herschel, Bait., K.H., 

 F.R.S,, &c., in a letter addressed to S. Hunter Christie. Phil. 

 Trans. 1843, vol cxxxiv. 



* Sir J. F. W. Herschel ; see also Researches on Light, by the 

 Author. 



