PROFESSOR AT KONIGSBERG 97 



colour. In order to refute Brewster's theory, or to answer 

 the question whether the colour of homogeneous light is 

 altered by coloured media or no, the validity of his experi- 

 ments had in the first instance to be tested. Helmholtz 

 found that Brewster had overlooked the false light cast 

 over the observer's field of vision by the slight turbidity 

 inevitable in transparent bodies. He shows that the altera- 

 tions of colour which Brewster had remarked are due partly 

 to impurities in the glass of the prism and to irregularities 

 in the polishing of its faces, partly to multiple reflections 

 at the surfaces of the prism and of the coloured media, 

 as well as to dispersion of light in the eye itself; further, 

 such alterations of colour may depend on contrast effects, 

 excited by the luminosity of the spectral colours; while, 

 finally, the colours of the spectrum excite a different im- 

 pression with different intensities of light. Thus he refuted 

 the theories of Brewster by a long series of conclusive experi- 

 ments. 



These investigations were just concluded when Helmholtz 

 delivered his Inaugural Lecture, for which he selected a 

 subject standing in the closest relation with his previous 

 work, namely, a general discussion of the mode in which 

 our sense-perceptions correspond with the objects perceived: 

 a question that led him far into problems of the theory of 

 knowledge, and at the same time struck the note of his 

 further researches in physiology and physics. 



After a masterly exposition of the undulatory theory of 

 sound and light, and a defence of Newton's theory of colour 

 against that of Brewster on the grounds above stated, he 

 emphasizes the simplicity and apparent unity of a compound 

 colour-sensation. He refers the opposition of Goethe (and 

 after him of the whole of the Hegelian school) to the idea 

 of the composite nature of white light to the idiosyncracies 

 of the poet-genius, who held it to be his highest function to 

 insist on the adequacy of sense phenomena, and assumed 

 that the same directness of perception should be possible 

 in the intellectual world. 



In order to obtain a less superficial view of optical pheno- 

 mena, it is necessary to recognize the relation borne by 

 luminous sensations to the objects sensed; and Helmholtz 



