OR COLOUR BLINDNESS. 183 
tend to confirm this testimony. The opinion of Dalton cannot 
therefore be adopted. 
On the contrary, Goethe* thinks that to explain daltonism it 
is sufficient to admit that those affected do not see blue, but in 
its place a weak purple, a rose-colour and a light and pure red. 
This led him to give the name of Akyanoblepsis to this affection. 
It is to be presumed that if he had known the different cases 
which I have described, he would not have advanced an hypo- 
thesis which suits so little with many of them. 
All natural philosophers know that the limits of perceptible 
sounds vary between one ear and the other,—a fact which Wol- 
laston and Chladni have incontrovertibly demonstrated. Sir D. 
Brewster} supposes, by analogy, that in the cases of daltonism 
the eye is not impressed by the colours of one of the extremities 
of the spectrum. ‘“ The insensibility of some eyes to weak im- 
pressions of light, requires,” says he, “ no other explanation, than 
that either from original organisation, or some accidental cause, 
the retina of one person may be less delicate and less suscep- 
tible of luminous impressions than the retina of another, without 
being accompanied with any diminution of the powers of vision.” 
But the more recent researches of M. Savart having established 
the fact that in suitable circumstances the limit of the sounds 
perceived by the same ears may be considerably modified and 
extended, and nothing similar having been able to be proved in 
what concerns the defect, the cause of which we are investi- 
gating, it appears to me that this theory wants proofs{. It is 
known, moreover, that the sensibility of the sound eye is com- 
paratively much less than that of the ear, for if we represent the 
length of undulation in the air for the extreme red by 0?°:0000266 
(English measure), and for the extreme violet by O?°-0000167, 
we see that the relation of the corresponding vibrations is that 
of 1°59 to 1, a value a little below 1°6, which is that of the minor 
sixth, and consequently much lower than that of an octave. 
At a later period the illustrious natural philosopher of St. 
Andrews appears to have changed his opinion§; setting out 
* Goethe. Zur Farbenlehre, §§ 111 to 113. Wh. Nicholl also (Annals of 
Philosophy, No. 8, tome iii.) attributes it to the absence of sensibility for blue. 
+ Mackenzie, op. cit. Edinb. Journal, vol. vi. p. 141. [See Sir David 
Brewster’s remarks on this paragraph in the Philosophical Magazine for Au- 
gust 1844, p. 134.—Ebir. ] 
t See Sir D. Brewster in Phil. Mag., wbi supra. 
_ § Edinb. Journ, of Science, tome vii. p. 86. [See Sir D. Brewster’s reply 
in the Philosophical Magazine for Aug. 1844, p. 137.] 
