MEMOIR VI.] THE DIFFRACTION SPECTRUM. 97 



MEMOIR VI. "</ , ' v '.y . 



N^ X/V, ~*Jr 



ON THE DIFFRACTION SPECTRUM. (j^ J 



From the Philosophical Magazine, March, 1857. ^</ 



CONTENTS: Mode of obtaining the diffraction spectrum. The yellow is 

 in its middle ; it is a centre of chemical action. It is also the hottest 

 ray. Diffraction spectra by reflection. Cold lines. Dilatation of the 

 more refrangible rays in the prismatic spectrum, compression of the less 

 refrangible. Action of the diffraction spectrum on salts of silver. Use 

 of wave-lengths for spectrum division. 



M. EISENLOHR having published in PoggendorfFs An- 

 nalen for June and August, 1856, some researches on the 

 diffraction spectrum, from which it appeared that he was 

 not aware of the results I had obtained and published in 

 1844, 1 thought it well to call attention to them, and this 

 I did in a letter to the editors of the Philosophical Mag- 

 azine, which they published in their journal for March, 

 1857, and from which the following extracts are made: 



At first I obtained the diffraction spectrum very much 

 in the same manner that M. Eisenlohr has done, by pass- 

 ing a beam of light directed through a vertical slit by a 

 heliostat, and through a piece of ruled glass at twelve 

 feet distance. It was then received on an achromatic 

 lens of about four-feet focus, which gave a sharply de- 

 fined spectrum on a ground-glass or photographic sur- 

 face. Subsequently I found that it was much better to 

 silver the ruled surface with tin amalgam, in the manner 

 of a looking-glass, and thus employ a reflected spectrum. 

 This is far more brilliant than the transmitted one, and 

 the silvering acts so perfectly that the most minute fixed 

 lines may be seen. 



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