British Association for the Advancement of Science. Ill 



Spectrum, which I have been led to notice in the prosecution of 

 my inquiries into the action of the spectrum on paper, rendered 

 sensitive to the chemical rays by Mr. Talbot's process, or by 

 others of my own devising. 



The property in question is this : — that the extreme red rays, 

 (such, I mean, as are insulated from the rest of the spectrum by 

 a dark blue glass colored by cobalt, and which are not seen in the 

 spectrum unless the eye be defended by such a glass from the 

 glare of the other colors, ) not only have no tendency to darken 

 the prepared paper, but actually exert a contrary influence, and 

 preserve the whiteness of paper on which they are received, 

 when exposed at the same time to the action of a dispersed light 

 sufficient of itself to produce a considerable impression. I have 

 long suspected this to be the case, from phenomena observed in 

 taking photographic copies of engravings ; but having at length 

 obtained demonstrative evidence of the fact, I think this may 

 not be an improper opportunity to announce it. 



When a slip of sensitive paper is exposed to a highly concen- 

 trated spectrum, a picture of it is rapidly impressed on the paper, 

 not merely in hlack, but in colors^ a fact which I ascertained 

 nearly two months ago, and which observation of mine seems to 

 have been alluded to (though in terms somewhat equivocal) by 

 M. Arago, in his account of Daguerre's process. In order to un- 

 derstand what follows, it will be necessary to describe the colors 

 so depicted. The red is tolerably vivid, but is rather of a brick 

 color than of a pure prismatic red ; and what is remarkable, its 

 termination falls materially short of the visible termination of the 

 spectrum. The green is of a sombre, metallic hue ; the blue 

 still more so, and rapidly passing into blackness. The yellow is 

 deficient. The whole length of the chemical spectrum is not 

 far short of double that of the luminous one, and at its more re- 

 frangible end a slight ruddy or pinkish hue begins to appear. The 

 place of the extreme red, however, is marked by no color, thus 

 justifying so far the expression which M.- Arago is reported to 

 have used in speaking of my experiments, " Le rayon rouge est 

 seul sans action." 



It is impossible in this climate to form a brilliant and condensed 

 spectrum without a good deal of dispersed light in its confines ; 

 and this light, if the exposure of the paper be prolonged, acts, of 

 course, on every part of its surface. The colored picture is formed, 



