1880.] on Goethe's * Farhenlehre: 349 



in the ' Farbcnlchro,' had corrected the error, and proved the mixture 

 of blue and yellow lights to produce white. Any doubt that miglit 

 be entertained of Wiinsch's experiments — and they are obviously the 

 work of a careful and competent man — is entirely removed by the 

 experiments of Helmholtz and others in our own day. Thus, to sum 

 up, Goethe's theory, if such it may be called, proves incompetent to 

 account even approximately for the Newtonian spectrum. He refers 

 it to turbid media, but no such media come into play. He fails 

 to account for the passage of yellow into red and of blue into violet ; 

 while his attempt to deduce the green of the spectrum from the 

 mixture of yellow and blue, is contradicted by facts which were 

 extant in his own time. 



One hole Goethe did find in Newton's armour, through which ho 

 incessantly worried the Englishman with his lance. Newton had 

 committed himself to the doctrine that refraction without colour 

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

 telescopes must for ever remain imperfect, achromatism and refraction 

 being incompatible. The inference of Newton was proved by 

 Dollond to be wrong.* With the same mean refraction, flint glass pro- 

 duces a longer and richer spectrum than crown glass. By diminish- 

 ing 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 colours may be neutralized, 

 while a considerable residue of refraction continues in favour 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 elsewhere, 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 followers. But the error, which was a real 

 one, leaves Newton's theory of colours 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 

 colours. 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 bo 

 thereby refracted to the opposite wall. It was at first a very pleasing 

 divertisement, to view the vivid and intense colours produced thereby ; 

 but after a while applying myself to consider them more circumspectly, 

 I became surprised to see them in an oblong form, which according 



* Dollond was the son of a Huguenot. Up to 1752 he was a silk weaver at 

 Spitalfields; he afterwards became an optician. 



