62 GOETHE'S 'FARBENLEHRE/ 



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 his lance incessantly worried the Eng- 

 lishman. Newton had committed himself to the doc- 

 trine that refraction without colour was impossible. 

 He therefore thought that the object-glasses of tele- 

 scopes must for ever remain imperfect, achromatism 

 and refraction being incompatible. The inference of 

 Newton was proved by Dollond to be wrong. 1 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 spec- 

 trum 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 neutralised, while a con- 

 siderable 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 in- 

 terpretation that he lacks. He flaunts this error re- 

 garding achromatism incessantly in the face of Newton 

 and his followers. But the error, which was a real one, 



1 Dollond was the son of a Huguenot. Up to 1752 he was a 

 silk weaver at Spitalfields ; he afterwards became an optician. 



