121 Prof. A. M. Mayer on the History of Young's 



on the sense of sight might bo imitated by a mixture of colours 

 taken from different parts of the spectrum, notwithstanding 

 the omission of some of the rays naturally belonging to white 

 light. Thus if we intercept one half of each of the four prin- 

 cipal portions into which the spectrum is divided, the remaining 

 halves will still preserve, when mixed together, the appear- 

 ance of whiteness ; so that it is probable that the different 

 parts of those portions of the spectrum which appear of one 

 colour have precisely the same effect on the eye. It is certain 

 that the perfect sensations of yellow and of blue are produced 

 respectively by mixtures of red and green and of green and 

 violet light ; and there is reason to suspect that those sensations 

 are always compounded of the separate sensations combined ; at 

 least this supposition simplifies the theory of colours ; it may 

 therefore be adopted with advantage until it be found incon- 

 sistent with any of the phenomena ; and we may consider white 

 light as composed of a mixture of red, green, and violet only, in 

 the proportion of about two parts red, four green, and one 

 violet, with respect to the quantity or intensity of the sensa- 

 tions produced. 



" If we mix together in proper proportions any substances 

 exhibiting these colours in their greatest purity and place the 

 mixture in a light sufficiently strong, we obtain the appearance 

 of perfect whiteness ; but in a fainter light the mixture is grey, 

 or of that hue which arises from a combination of white and 

 black, black bodies being such as reflect white light, but in a 

 very scanty proportion. For the same reason, green and red 

 substances mixed together usually make rather a brown than 

 a yellow colour, and many yellow colours, when laid on very 

 thickly or mixed with black, become brown. The sensations 

 of various kinds of light may also be combined in a still more 

 satisfactory manner by painting the surface of a circle with 

 different colours in any way that may be desired, and causing 

 it to revolve with such rapidity that the whole may assume the 

 appearance of a single tint, or a combination of tints resulting 

 from the mixture of the colours. 



" From three simple sensations, with their combinations, we 

 obtain seven primitive distinctions of colours ; but the different 

 proportions in which they may be combined afford a variety of 

 tints beyond all calculation. The three simple sensations being 

 red, green, and violet, the three binary combinations are 

 yellow, consisting of red and green ; crimson, Of red and 

 violet ; and blue, of green and violet ; and the seventh in order 

 is white light composed of all three united. But the blue thus 



Young's account ; but we have found it impossible to convey in print clear 

 ideas of their tints. 



