150 Royal Society :-—— 
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 prumary 
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 strongl 
disposed to believe that he sees white as we do, for reasons which [ 
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- 
matic analysis of the fullest and most vivid yellows (those which 
excite the sensation of yellowness in the greatest perfection), as the 
colours of bright yellow flowers, or that of the yellow chromate of 
mercury, clearly demonstrates the fullness, richness, and brillianey 
of their colour to arise from their reflexion of the whole, or nearly 
the whole of the red, orange, yellow, and green rays, and the sup- 
pression of all, or nearly all the blue, indigo, and violet portion of the 
spectrum. On the hypothesis of an analysis of sensation correspond- 
ing to an analysis of coloured light, these facts would seem incom- 
patible with the simplicity of the sensation yellow, and it would 
appear impossible (on that hypothesis) to express them otherwise 
than by declaring red and green to be primary sensations, and yellow 
a mixture of them—a proposition which needs only to be understood 
to be repudiated—quite as decidedly as that the sensation of green- 
ness is a mixture of the sensations of blueness and yellowness, and for 
the same reason; the complete want of suggestion of the so-called 
simple sensations by the asserted complex ones. 
Mr. Maxwell’s assertion that blue and yellow do not make green, 
assuredly appears startling as contradictory to all common experience ; 
but the common experience appealed to is that of artists, dyers, and 
others in the habit of mixing natural colours as they are presented to 
