164 PROFESSOR WARTMANN ON DALTONISM, 
Troughton), who had lost an eye by an accident, and whose 
other eye perceived a sensation of brightness in all parts of the 
spectrum, but distinguished in it only two colours, blue corre- 
sponding to the most refrangible rays, and yellow to those which 
are the least so. Experiments in confirmation have been made 
by means of polarized light, which traversed a moveable lamina 
of mica, and which gave simultaneously two colours, the one 
complementary of the other, and whose coloration differed ac- 
cording to the inclinations of the crystal. According to Mr. 
Whewell, all the male members of the Troughton family are 
similarly affected*. 
The most interesting investigation to be noticed on this sub- 
ject is that of the illustrious Dalton, who has described the 
affection which we are considering in his own case and in that 
of some of his pupils. The vast extent of Dr. Dalton’s know- 
ledge is a guarantee of the fidelity and care he has exercised in 
analysing his sensations, and renders the sketches of them which 
he has published} very valuable. Dr. Dalton sees in the solar 
spectrum three colours only, yellow, blue and purplet. The 
first two form a contrast, the last two seem to differ rather in 
degree than specifically. Rose colour seen by day appears sky 
blue a little weakened ; in artificial light it takes an orange or 
yellowish tint, which contrasts strongly with the blue. Crimson 
is, by day, a dirty blue, and crimson wool a purple. Red and 
scarlet become by candle-light more brilliant and more vivid. 
By daylight Dalton perceives no difference between the red of 
a stick of sealing-wax and grass green; he calls the carnation of 
a ruddy complexion dark blue, and he assorts crimson with the 
colour of bright red, wine or mud, rose colour with light blue, 
brown with red, and gray with green. Lastly, on all points 
where he has a peculiar perception, the difference is less by 
candle-light than in sun-light. 
Mr. Whewell relates §, that having asked Dalton to what ob- 
* Atheneum, loc. cit. 
+ Extraordinary facts relating to the Vision of Colours, with observations, 
by Mr. John Dalton; read Oct. 31st, 1794.—Memoirs of the Literary and 
Philosophical Society of Manchester, vol. v. part 1. p. 28. 1798. 
{ Sir D. Brewster affirms, on the contrary, in his ‘ Letters on Natural Magic,’ 
that according to his observations, Dalton saw the whole entire spectrum, but 
that the red portion appeared to him yellow. (Page 27 of the French trans- 
lation.) —E. W. [There is no contrariety whatever between Sir D. Brewster’s 
affirmation and Dr. Dalton’s own statement; Dr. Dalton has never stated that 
the spectrum he saw was shorter than the spectrum seen by others.—Ep. ] 
'§ Athenzeum, loc. cit. 
