THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MARCH 1857. 



XXIII. On the Diffraction Spectrum. — Remarks on M.Eisenlohr's 

 recent Experiments. By John W. Draper, M.D., Professor 

 of Chemistry and Physiology in the University of New York. 



To the Editoi-s of the Philosophical Magazine and Journal. 



Gentlemen, 



WILL you permit me to direct the attention of your readers 

 who may have been interested in the beautiful experi- 

 ments published by M. Eiseulohr in PoggendorflF's Annalen for 

 June and xiugust 1856, to some researches on the same subject 

 which I made about fourteen years ago, and which were pub- 

 lished in 1844 in a woi'k " On the Forces which produce the 

 Organization of Plants.^' It is a matter of regret to me that 

 M. Eisenlohr does not appear to have been aware of those results, 

 or doubtless he would have perceived that some of the facts which 

 he supposes to be new have been known in America for many 

 years. 



As the work to which I have referred is now out of print, you 

 will perhaps indulge me with space enough to give a short abs- 

 tract of those portions of it bearing on the diffraction or inter- 

 ference spectrum . 



I obtained, at first, that spectrum very much in the same man- 

 ner that M. Eisenlohr has done, by passing a beam of light 

 directed through a vertical slit by a heliostat through a piece 

 of ruled glass at 12 feet distance, and then received it on an 

 achromatic lens of about 1 feet focus, which gave a sharply- 

 defined spectrum on a ground glass or photographic surface. 

 Subsequently I found that it was much better to silver the ruled 

 surface with tin amalgam, in the manner of a looking-glass, and 



Phil. Mag. S. 4. Vol. 13. No. 85. March 1857. M 



