XXXV.] COLOURS AND THEIR COMBINATION. Ill 



So that auy one of the following pairs of colours, when 

 combined, will make white : — 



Eed and bluish green. 

 Orange and azure. 

 Yellow and indigo. 

 Yellowish green and violet. 

 Green and purple. 



It is to be observed that, in such experiments as these, what 



we must do is to combine the colours themselves ; and this is to How to 



be done by letting rays of the two colours which we wish to combine 

 , . p 1, 1 p n • colours. 



combine fall on the same spot of white paper. Mixture of 



colouring stuffs will not give the same results.^ 



It is also to be observed, that the white produced by the Whites 



combination of any one pair of complementaries, though the F'oj;l^^ced 



same in appearance as that produced by the combination of any combina- 



other pair, is not really the same. The difference is shown by ^}?^ . 



decomposing any white ray with the prism, which will separate pairs of 



it into its differently-coloured constituents ; iust as the com- comple- 



"' . ' '' mentanes 



■mon white light of day is decomposed by the prism into all are 



the colours of the spectrum. Thus, the white which is com- ^I'^^'^^^'y 

 ^ . dineveiit. 



pounded of red and bluish green may be separated by the prism 

 back into red and bluish green ; and the white which is com- 

 pounded of yellow and indigo may be in like manner separated 

 back into yellow and indigo. Thus, two whites may be, to use 

 Professor Clark Maxwell's expression, optically different though 

 chromatically alike. The white light of day is of course a 

 mixture of the whites which are compounded of all the pairs of 

 complementaries in the spectrum. 



that the failure to find homogeneous complementaries (that is to say, com- 

 plementaries in the spectrum) for the colours from gi-een-yellow to greeji- 

 blue is due merely to the faintness of the purple tints, among which by 

 the theory their complementaries shoiild be found. 



^ If we mix two or more colouring materials together, the colour of the 

 mixture is due to those rays which are not absorbed by any of the mate- 

 rials ; so that the more ingi'edients we put into our mixture the more rays 

 are absorbed, and the nearer we come to the total suppression of light, 

 which is blackness. Hence the necessity, in water-colour painting, of 

 obtaining brilliancy, if possible, by a single wash of colour. If on the 

 contrary we mix the colours themselves in the way described in the text, 

 the more colours we add the more light we get. It is thus obvious that 

 the two methods of mixing will not give the same results. For the remark 

 about water-colours, see Sir John Herschel, in Good Words, August 1865. 



