113 British Association for the Advancement of Science. 



therefore, on a ground not purely white, but rendered dusky over 

 its whole extent, with one remarkable exception, viz. in that spot 

 where the extreme red rays fall, the whiteness of which is pre- 

 served, and becomes gradually more and more strikingly apparent 

 the longer the exposure and the greater the consequent general 

 darkening of the paper. 



The above is not the only singular property possessed by the 

 extreme red rays. Their action on paper already discolored by 

 the other rays is still more curious and extraordinary. When 

 the spectrum is received on paper already discolored slightly by 

 the violet and blue rays 07ily, they produce, not a white, but a red 

 impression, which, however, I am disposed to regard as only the 

 commencement of a process of discoloration, which would be 

 complete if prolonged sufficiently. For I have found that if in- 

 stead of using a prism, a strong sunshine is transmitted through 

 a combination of glasses carefully prepared, so as to transmit ab- 

 solutely no ray but that definite red at the extreme af refrangi- 

 bility, a paper previously darkened by exposure under a green 

 glass has its color heightened from a sombre neutral tint to a 

 bright red ; and a specimen of paper rendered almost completely 

 black by exposure to daylight, when exposed for some time under 

 the same glass, assumed a rich purple hue, the rationale of which 

 effect I am disposed to believe consists in a very slow and grad- 

 ual destruction, or stripping off as it were, of layers of color de- 

 posited or generated by the other rays, the action being quicker 

 on the tints produced by the more refrangible rays in proportion 

 to their refrangibilities. 



It seems to me evident that a vast field is thus opened to fur- 

 ther inquiries. A deoxydizing power has been attributed to the 

 red rays of the spectrum, on the strength of the curious experi- 

 ments of Wollaston on the discoloration of tincture of guaiacura, 

 which ought to be repeated ; but in the sensitive papers, and still 

 more in Daguerre's marvellous ioduretted silver, we have re-agents 

 so delicate and manageable, that every thing may be expected 

 from their application. J. F. W. Herschel. 



Slough, August 28, 1839. 



On the effects of lightning in three of Her Majesty'' s ships, by 

 Mr. Snow Harris. Mr. H. showed from the facts which he had 

 collected, that were the masts of ships made perfectly good con- 



