3 i2 THE POPULAR SCIENCE MONTHLY. 



GOETHE'S FAEBENLEHEE. (THEOEY OF COLOES.*) 



By Professor JOHN TYNDALL, F. E. S. 



II. 



ONE hole Goethe did find in Newton's armor, through which he 

 incessantly worried the Englishman with his lance. Newton 

 had committed himself to the doctrine that refraction without color was 

 impossible. He therefore thought that the object-glasses of telescopes 

 must for ever remain imperfect, achromatism and refraction being in- 

 compatible. This inference was proved by Dollond to be wrong.f 

 With the same mean refraction, flint-glass produces a longer and richer 

 spectrum than crown-glass. By diminishing the refracting angle of 

 the flint-glass prism, its spectrum may be made equal in length to that 

 of the crown-glass. Causing two such prisms to refract in opposite 

 directions, the colors may be neutralized, while a considerable residue 

 of refraction continues in favor of the crown. Similar combinations 

 are possible in the case of lenses ; and hence, as Dollond showed, the 

 possibility of producing a compound achromatic lens. Here, as else- 

 where, Goethe proves himself master of the experimental conditions. 

 It is the power of interpretation that he lacks. He flaunts this error 

 regarding achromatism incessantly in the face of Newton and his fol- 

 lowers. But the error, which was a real one, leaves Newton's theory 

 of colors perfectly unimpaired. 



Newton's account of his first experiment with the prism is for ever 

 memorable. " To perform my late promise to you," he writes to 

 Oldenburg, "I shall without further ceremony acquaint you that in 

 the year 1666 (at which time I applied myself to the grinding of optick- 

 glasses of other figures than spherical) I procured me a triangular glass 

 prism, to try therewith the celebrated phenomena of colors. And in 

 order thereto, having darkened my chamber, and made a small hole in 

 my window-shuts, to let in a convenient quantity of the sun's light, I 

 placed my prism at its entrance, that it might be thereby refracted to 

 the opposite wall. It was at first a very pleasing divertisement, to 

 view the vivid and intense colors produced thereby ; but after a while 

 applying myself to consider them more circumspectly, I became sur- 

 prised to see them in an oblong form, which, according to the received 

 laws of refractions, I expected should have been circular. They were 

 terminated at the sides with straight lines, but at the ends the decay 

 of light was so gradual that it was difficult to determine justly what 

 was their figure, yet they seemed semicircular. 



* A discourse delivered in the Royal Institution of Great Britain, on Friday evening, 

 March 19, 1880. 



} Dollond was the son of a Huguenot. Up to 1752 he was a silk-weaver at Spital- 

 fields ; he afterward became an optician. 



