Section III. 1891. [ 49 J Trans. Roy. Soc. Canada. 



VII. — Newtoii's use of the Slit and Lens In formiiuj a pure spectrum. — Common error 

 concerning this. — Effectiveness of Newlou's method hi shoioing the dark lines 

 on a screen. 



By Alexander Johnson, M.A., LL.D., Dublin, Professor of Mathematics and Natural 



Philosophy, McGill University, Montreal. 



(Read May 27, 3S91.) 



I. G-ENERAL. 



The object of the following paper is threefold : first, to eall attention to an error 

 which is spreading- through scientific books and does injustice to Newton's work in 

 optics ; secondly, to point out the extraordinary fact that not only Newton's method but Ids 

 actual experiments ivere fully su/ftcient, ivith ordinary tuck, to show the dark lines in the solar spec- 

 trum, while, as we know, he did not see them ; thirdly, to suggest that a republication of 

 the last edition of Newton's " Opticks " is of sufHcient value to students of science of the 

 present day to justify the outlay. The book is not easy of access, yet much may be 

 learned from the account of the original experiments ; moreover, when one writer, not 

 having the original at hand, copies from another statements concerning it, error easily 

 arises and is readily propagated. It would be most fitting that Newton's own university 

 should undertake this republication. 



The error I wish to point out is the statement that Newton never used the slit in 

 producing the spectrum, and therefore could not have produced homogeneous light, that 

 is, as I take it, sufficiently homogeneotts to show the dark lines in the solar spectrum. 



The following quotations may be submitted : — 



Roscoe ("Spectrum Analysis," 1869, p. 22) says: "The first person who observed 

 these dark lines was Dr. WoUaston. Newton did not observe them, and for the good rea- 

 son that he allowed the light to fall on the prism from a round hole in the shutter." — 

 " If he had allowed the light to pass through a fine vertical slit, and if this slit of light, 

 if we may use such a term, had then fallen upon the prisms, placed so that the edge of 

 the refracting angle is parallel to the slit, he would have observed that the solar spectrum 

 is not continitous, but broken up by permanent dark lines." 



Lockyer (" The Spectroscope," 1873, p. 18) says : " It is very curious, however, that 

 Newton, although he made many experiments on prisms, really omitted one of the most 

 important points." — " Newton made a round hole in a shutter for his experiments, 

 but we now know he ought not to have done that : he ought to have made a slit ; but 

 this did not come out until 1802, w^hen Dr. "Wollaston, by merely using a slit instead of a 

 round hole, made a tremendous step in advance." 



Sec. Ill, 1891. 7. 



