BOOK IX 



PHYSICAL OPTICS. 



Photography. 



I HAVE, at the end of Chapter xi., stated that the theory of whicfl 

 I. have endeavored to sketch the history professes to explain only 

 the phenomena of radiant visible light ; and that though we know 

 that light has other properties for instance, that it produces chemical 

 effects these are not contemplated as included within the domain of 

 the theory. The chemical effects of light cannot as yet be included 

 in exact and general truths, such as those which constitute the undu- 

 latory theory of radiant visible light. * But though the present age 

 has not yet attained to a Science of the chemistry of Light, it has 

 been enriched with a most exquisite Art, which involves the principles 

 of such a science, and may hereafter be made the instrument of bring- 

 ing them into the view of the philosopher. I speak of the Art of 

 Photography, in which chemistry has discovered the means of pro- 

 ducing surfaces almost as sensitive to the modifications of light as the 

 most sensitive of organic textures, the retina of the eye : and has 

 given permanence to images which in the eye are only momentary 

 impressions. Hereafter, when the laws shall have been theoretically 

 established, which connect the chemical constitution of bodies with 

 the action of light upon them, the prominent names in the Prelude to 

 such an Epoch must be those who by their insight, invention, and 

 perseverance, discovered and carried to their present marvellous per- 

 fection the processes of photographic Art : Niepce and Daguerre in 

 France, and our own accomplished countryman, Mr. Fox Talbot. 



Fluorescence. 



As already remarked, it is not within the province of the undulatory 

 theory to explain the phenomena of the absorption of light which take 

 place in various ways when the light is transmitted throuo-h various 



