Bauhaus-Universität Weimar

Titel:
Dictionary of philosophy and psychology including many of the principal conceptions of ethics, logics, aesthetics ... and giving a terminology in English, French, German and Italian, vol. 2 [lead-zwing]
Person:
Baldwin, James Mark
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29448/828/
VISION 
It is still Parinaud who first pointed out 
that it is only the achromatic constituent of 
the sensation that is affected by this change. 
In proportion as the blues become relatively 
brighter, they become also less saturated, 
and, still more, the greens, as they become 
bright, become finally wholly uncoloured. 
The reinforcement occurs, that is to say, not 
for colour in itself, but only by way of mixing 
in more white or grey. (This is sufficient, 
doubtless, to account for the fact that in 
a very faint spectrum blue is not seen at all ; 
the spectrum looks simply red or green, and 
this in spite of the fact that the Purkinje 
phenomenon is usually considered to consist 
exactly in the brightening of this colour. 
The blue becomes, in fact, so much overlaid 
with the white constituent furnished by the 
rods that it is no longer visible as blue.) 
Again, this fading out into an achromatic 
sensation before becoming wholly extinguished 
—which is what the Purkinje effect really 
consists in, the change in brightness being an 
attendant phenomenon—does not occur in 
the fovea. If a spot of coloured light is so 
minute as to throw its image upon the fovea 
only, then, however faint it is, if it is seen 
at all, it is seen in its true colour, not first 
by means of its colourless constituent. This 
has been doubted in the case of blue, but it 
has been confirmed by König and v. Kries, 
and recently again by Fick. (The case is less 
clear for yellow.) 
The argument in favour of a difference in 
function of rods and cones was thus already 
in the hands of Parinaud exceedingly strong. 
It happened, however, that it remained com¬ 
pletely overlooked, and the several facts 
noticed by him were rediscovered by other 
observers—the fact that colours seen with the 
fovea lack the preliminary achromatic stage 
of rod-vision by König in 1894 (‘Ueber den 
menschlichen Sehpurpur, &c.,’ Sitzber. Akad. 
Wiss. Berlin vom 21. Juni 1894); König 
did not, however, uphold the theory here in 
question—he regarded the cones as catoptric 
instruments, viz. as condensers for throwing 
light upon the cells of the pigment epi¬ 
thelium, where he supposed all colour pro¬ 
cesses except that for blue to take place) ; 
the normal night-blindness of the fovea—that 
is, the fact that the extremely faint lights 
which the rod-adaptation exists for the sake 
of enabling us to see are wholly invisible in 
the fovea—and also the total blindness in 
the fovea of some of the congenitally totally 
colour-blind, a little earlier by Ladd-Franklin 
(ibid., 589, and Broc. Amer. Psychol. Assoc., 
1894); the absence in the fovea of any 
change in the relative brightness-values of 
different spectral lights, first made certain by 
v. Kries in 1900, by means of colour equa¬ 
tions, in which each member is made alike in 
colour, although different in light-ray con¬ 
stitution (Zeitsch. f. Psychol., xxiii). This 
theory of the probable difference in function 
of the rods and cones had already been made 
(1892) the ground-work of the molecular dis¬ 
sociation colour-theory (or theory of the 
developed photochemical substances : Zeitsch. 
f. Psychol., iv, 18 9 2 ) ; it is so strongly suggested 
by the fact that night-animals—owls, rats, 
moles—have retinae almost wholly deficient in 
cones, but containing rods which are exceed¬ 
ingly rich in the visual purple, that it would 
have been simply accepted ever since its first 
proposal by Max Schultze had it not been that 
it was wholly contradictory to the reigning 
Young-Helmholtz colour-theory, v. Kries, 
who is a warm defender of the theory (but 
in the form that the rods are altogether a 
‘ darkness-apparatus/ and that, although they 
outnumber the cones twenty to one, they are 
wholly functionless in an ordinary illumina¬ 
tion), apparently holds now to the belief that 
there are two sorts of white—one physio¬ 
logical and brought about by a photochemical 
dissociation in the rods, the other psychical 
and due to a mental reconstitution of an 
even red-green-blue sensation into a sensation 
of indistinguishable quality from the first. 
From the laboratory of v. Kries have issued 
a number of important investigations which 
have had for their object the establishment 
of the disjunction of function of the rods and 
the cones ; to refer to the theory as the 
v. Kries theory is, however, a mistake1. 
If the rods furnish colourless vision only, 
is it not possible that the vision of the 
totally colour-blind is vision solely by the 
rods ? If that were the case, these defectives 
should be totally blind in the fovea, where 
there are only cones. That was found to be 
the case in the first instance of the defect 
which was tested in this regard (Sitzber. 
Akad. Wiss. Berlin vom 21. Juni 1894, 593). 
A number of cases which have been examined 
since (Hess, Hering, Pflüger) seem not to 
have had this defect ; but the case of Uhthoff, 
which was first announced by him to be 
1 And still more so to call it (Abelsdorff) the 
Schultze-König theory. Aside from the question of 
priority, the theory of König is not even the same as 
that of Schultze, either for the cones or for the rods.
        

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