OR COLOUR BLINDNESS. 185 
four, in what relates to their physiological influence upon the eye. 
It is consequently sufficient to admit that the nerves are more 
or less sensible to the heating action of light, in order to under- 
stand that in all possible cases the yellow, which is the most lu- 
minous colour, will be recognised entirely and exactly, whilst 
all the other colours will only be apparent by opposition; this 
is the reason why green and red are not distinguished, and are 
often confounded with blue and yellow. We know, by a mul- 
titude of experiments, that the eye is the least sensible to red 
and green*. 
Lastly, Sir J. W. Herschel+ and Dr. Elliotsont{ are of opi- 
nion that this malady of the vision must be attributed to a de- 
fect in the sensorium itself, which renders it incapable of appre- 
ciating exactly the differences which exist between the rays, and 
on which their particular colours depend. This view is evidently 
only a formula of facts; it is the only one against which no ob- 
jection lies. It has been adopted both by Mr. Harvey § to ac- 
count for cases which he has described, and by Dr. Young, who, 
setting out from the observations of Darwin, admits the absence 
or the paralysis of the fibres of the retina designed for the per- 
ception of red ||. 
Whatever, in conclusion, are the circumstances which have 
engendered or which characterize this remarkable affection, it is 
of the number of those which the Esculapian art is unable to 
destroy. The most celebrated oculists, such as the Mackenzies {, 
Jiingkens**, Maunoirs +}, &c., are unanimous on this point. 
There nevertheless exists a very easy means of rectifying, to a 
certain extent, the error of the appellation of colour. This means 
consists in examining coloured objects through a transparent 
medium, as a glass or a liquid, of a certain known tint. Suppose 
this tint red; the impression of a green body and of a red body, 
* [This is not the case. When the sensibility of the eye is diminished, red 
is the first colour that disappears, and the green is left.— Ep. ] 
+ J. W. Herschel. Op. cit. p. 434, § 507. 
t Froriep’s Neue Notizen, No. 247, 1839. 
§ Harvey. Edinb. Phil. Trans., vol. x. p. 258. Edinb. Journ. of Science, 
vol. v. p. 114. Bibl. Univ., tome xxxv. p. 84. 
|| Phil. Trans., vol. Ixxvi. p. 344. 
§| Mackenzie. Op. cit. 
** J.C. Jiingken, M.C., Dr. und Prof. der Heilkunde. Die Lehre von den 
Augenkrankheiten, ein Handbuch zum Gebrauche bei Vorlesungen und xum 
Selbstunterrichte fiir angehende Aertze. Berlin, 1832, 8yo. p. 841. ss. 
+t Verbal communication. 
VOL, IV. PART XIII. oO 
