M. H. Helmholtz on the Theory of Compound Colours. 521 



was compared by Newton to the intervals of the scale. Besides 

 this he must have been content with very incomplete apparatus, 

 and could therefore make but few observations on the artificial 

 union of two or more prismatic colours ; the results of these 

 seemed, on the whole, to correspond with those obtained from the 

 mixture of coloured substances. Besides these experiments he 

 made others on the mixture of coloured powders. 



Newton always obtained his spectra from sunlight, and did 

 not apply the methods necessary for the complete separation of 

 the differently coloured rays ; hence it is that he did not ob- 

 serve the lines of Fraunhofer in the spectrum. Wollaston* 

 was the first to obtain a spectrum so pure as to permit of a few 

 of these lines being seen in it. Through a very good flint-glass 

 prism he looked with his naked eye at a fine slit, tlu-ough which 

 diffused daylight entered, and saw, what indeed under these cir- 

 cumstances may always be observed, four well-defined coloured 

 bands in the spectrum ; red, yellow-green, blue, and violet. 

 The transitions from reddish orange through orange and yellow 

 into green-yellow, from green into blue, and from blue into 

 violet, arc so speedy in the flint-glass spectrum, that without the 

 help of a magnifying telescope they entirely escape the eye. In 

 this case the lines G and H of Fraunhofer bovnid violet very 

 sharply on both sides. The transition from green to blue is 

 marked by the lines b and F, and the nari'ow strip of pure yellow 

 being, in diffused daylight, very feebly luminous, it recedes in 

 the presence of the stronger red and green, so that these two 

 colours appear immediately contiguous. Wollaston therefore 

 assumes four primitive colours ; red, green, blue, and violet. 



Thomas Young accepted WoUaston's description of the spec- 

 trum, and altered to correspond with it his theory of colours, 

 which first assumed the three primitive colours generally recog- 

 nized, red, yellow, and blue, in the place of which he now set 

 red, green, and violet ; this necessitates the belief that he was 

 aware of the fact that from prismatic red and green yellow may 

 be obtained, and from prismatic green and violet blue. The 

 theory of Young bcfoi'c mentioned is important, inasmuch as in 

 it a definite physiological significance is assigned to the three 

 primitive colours, lie assumes that the particles which lie upon 

 the surface of the retina are capable of peculiar vibrations, and 

 that at each place particles exist possessing three different times 

 of vibration corresponding to the velocities of the oscillations of 

 the three primitive colours, violet, green, and red, which are to 

 each other in the ratios of 7, 6, and .5. If the number of vibra- 

 tions of a luminous ray were 5, it would only act upon the nerves 

 capable of the sensation of i*ed; if tlic number were 5',, the red 

 * Pliilosophical Transactious, 1H02, Pt. 2. p. .S78. 



