Prof. Grassmanu on the Theory of Compound Colours. 255 



In like manner we only distinguish the greater or less intensity 

 of a homogeneous colour. But even for the difference of the 

 individual homogeneous colours we have a mathematically deter- 

 minable standard, which is furnished most perfectly by the 

 duration of vibration corresponding to each colour; even in 

 ordinary speech, this difference has been very suitably distin- 

 guished by the expression tint. Thus, in a homogeneous 

 colour we can distinguish both its intensity and its tint. 

 If now a homogeneous colour be mixed with colourless light, 

 the impression of colour becomes weakened by this inter- 

 mixture. Ordinary language is rich in terms by which this 

 difference is characterized ; the expressions, intense, deep, pale, 

 dull, whitish, which are added to the names of colours, may 

 serve as instances. The scientific terminology, which must be 

 substituted for this popular nomenclature, arises spontaneously 

 from what has been above stated, as every impression of colour 

 of this kind may be analysed into three mathematically deter- 

 minable elements, — the tint, the intensity of the colour, and the 

 intensity of the intermixed white. The various tints form a 

 continuous series of such a kind, that when we start from one 

 colour of this series and proceed forward, we finally arrive at 

 the original colour. One cii'cumstance, however, must not 

 be left unnoticed here, namely, the difficulty of obtaining homo- 

 geneous red light, which forms the transition between the violet 

 and red of the ordinary spectrum, and which can only be pi'O- 

 duced by the prism under i-emarkably favoui-able circumstances 

 (on a bright summer's noon)*. This outermost colour of 

 the spectrum, which may be equally well regarded as extreme 

 red or extreme violet, I will call purple. If now we regard any 

 compounded light, the eye can distinguish in it only the three 

 elements above mentioned ; that is to say, every impression of 

 light may be imitated by mixing a homogeneous colour of a 

 certain intensity with colourless light of a certain intensity. 

 Thus in every sensation of light we have to distinguish three 

 things, — the intensity of the colour, the tint, and the inten- 

 sity of the intermixed colourless light. It would be easy to 

 make an apparatus by means of which we should be enabled 

 to determine every colour according to these three elements. 

 In order to give an idea of this, let us suppose two white 

 screens of similar nature moveable upon a hinge, in such a 

 manner that the white side of the screens shall be upon the 

 outside of the angle formed at the hinge ; let a divided circle 

 be also ])iovided to measure this angle. The coloured light to 

 be examined is now allowed to fall upon one of these screens in 



* See Poggendor£f'B Annalen, vol. xiii. p. 441. 



