OR COLOUR BLINDNESS. 161 
tion of Chromato-pseudopsis. We distinguishes five classes of 
them :— 
1. That of persons in whom the sense of colours is almost 
completely wanting, and who, in place of the elementary colours 
yellow, red and blue, see only different degrees of white and black. 
2. That of persons who also distinguish yellows :—external 
objects appear to them coloured with shades which generate the 
different mixtures of yellow, of white and of black. 
3. That of persons who not only see yellow, but are besides 
capable of a particular perception, and the same for blue and for 
red; these are the Akyanopes of Goethe. 
4. That of persons destitute solely of the perception of red, 
which appears to them ash gray. 
5. That of individuals who distinguish all colours, but not in 
a decided manner ; instead of being able to distinguish the mix- 
ture of two colours they never see but one of them. 
Prof. Purkinje divides daltonism into four varieties, which 
he names Achromatopsis, Chromatodysopsis, Akyanoblepsis, and 
Anerythroblepsis. The first two relate more to the intensity, the 
others to the nature, of the imperfection*. The same skilful 
physiologist has remarked that the latter also occurs in eyes in 
the normal state, when, the visual axis having a determinate di- 
rection, any coloured object is slowly introduced into the field of 
vision from its most outward boundary 7. 
The various ancient and recent observations which J have to 
relate appear to me to confirm the division of M. Seebeck, rather 
than that of MM. Szokalski and Purkinje. It would be very 
advantageous to make a strict classification of the daltonians, 
because for each category means might be offered at once sim- 
* Encyclopedisches Worterbuch der medicin. Wissenschaften, herausgegeben 
von den Professoren der medicin. Facultat zu Berlin, Bd. i. p. 259. 
+ Beobachtungen und Versuche zur Physiologie der Sinne, Bd. ii. p. 15. We 
also know that in certain circumstances both eyes do not see the same colours, 
if one of them receives laterally a bright-light, from which the other is pro- 
tected by a screen. ‘The following, among others, is an experiment not men- 
tioned in our treatises, and which would nevertheless deserve to be generally 
known. I take it from the Positions de Physique of M. Quételet*:—“ Hold a 
leaf of white paper at a foot distance from you, and look at an object placed 
further off, but so as still to see the paper, which appears double. The flame 
of a taper is then to be brought laterally near to one eye, and to be prevented 
by a screen from acting upon the opposite eye; the paper then appears red to 
the last eye and green to the first; it is white at the parts where the two images 
encroach upon one another. On rapidly bringing the light from the opposite 
side, you see the phenomena reproduced in an inverse direction: that which 
was red becomes insensibly green, and vice versd.”—Tome iii. p. 175, 2° edit. 
* [See an account of this experiment of Mr. Smith of Fochabers in the Phil. 
Mag., Series 3, vol. i. p. 249, and vol. ii, p. 168.—Ep. ] 
VOL. IV. PART XIII. M 
