74 



might suggest to Wollaston and Young, I cannot persuade myself 

 that either of them recognized the sensation of greenness as a con- 

 stituent of the sensations they received in viewing chrome yellow, or 

 the petal of a Marigold on the one hand, and ultramarine, or the 

 blue Salvia on the other ; or that they could fail to recognize a certain 

 redness in the colour of the violet, which Newton appears to have 

 had in view when he regarded the spectrum as a sort of octave of 

 colour, tracing in the repetition of redness in the extreme refrangible 

 ray, the commencement of a higher octave too feeble to affect the 

 sight in its superior tones. Speaking of my own sensations, I 

 should say that in fresh grass, or the laurel- leaf, I do not recognize 

 the sensation either of blue or of yellow, but something sui generis ; 

 while, on the other hand, I never fail to be sensible of the presence 

 of the red element in either violet, or any of the hues to which the 

 name of purple is indiscriminately given ; and my impression in this 

 respect is borne out by the similar testimony of persons, good judges 

 of colour, whom I have questioned on the subject. 



I would wish, then, to be understood as bearing in mind this 

 distinction when speaking of the composition of colours by the super- 

 position of coloured lights on the retina. It seems impossible to 

 reason on the joint or compound sensation which ought to result 

 from the supraposition in the sensorium of any two or more sensa- 

 tions which we may please to call primary ; so that if, following 

 common usage, I speak in what follows of red, yellow, and blue (or 

 in reference to Young's theory of red, green, and violet) as primary 

 colours, I refer only to the possibility of producing all coloured sensa- 

 tions by the union on the retina of different proportions of lights, 

 competent separately to produce those colours, which is purely a 

 matter of experience. 



It is necessary to premise this, when I remark that I by no means 

 regard as a logical sequence Mr. Pole's conclusion in 15, that 

 because he perceives as colours only yellow and blue, therefore the 

 neutral impression resulting from their union must be that sensation 

 which the normal-eyed call green. On the contrary, I am strongly 

 disposed to believe that he sees white as we do, for reasons which I 

 am about to adduce. 



Mr. Maxwell has lately announced his inability to form green by 

 the combination of blue and yellow. On the other hand, the pris- 



