434 PROFESSOR LUDWIG MOSER ON VISION, 
stance can assume under the influence of light, and then the | 
supposition that all colours act in the same manner, is quite un- 
tenable. It would be difficult to make experiments on this sub- 
ject, as it is difficult to operate with pure colours; but we must 
accede to the hypothesis that different colours act differently. 
There certainly is in one case a similar action in all colours, yellow 
and green included, viz. when they act for a length of time they 
then endow the iodide of silver with the power of condensing 
mercurial vapours, and by still continued action they blacken it. 
We have here then decidedly the same action in all the colours 
of the spectrum, and it is a question whether or not anything 
similar takes place in the eye. 
If there be any convincing proof of the correctness of my view 
concerning the process of vision, it certainly is the fact that ex- 
actly the same thing takes place with the eye, viz. that by con- 
tinued action all difference of colour vanishes, and what remains 
of them is merely a general impression of light. This is partly 
seen when we attempt to look at a brightly coloured object for 
a length of time, on which more will be said in the next part of 
this paper. But a clear evidence of the correctness of this view 
is an experiment for which we are indebted to Brewster, the 
result of which may be easily confirmed. 
If we observe for a length of time the spectrum of the flame 
of a candle through a prism, first of all the red and green parts 
vanish, and also alittle of the blue; if we still continue to look 
at it without moving the eyes, the yellow disappears, and passes 
into white; so that at last, instead of the prismatic colours, we 
only see a uniform white, lengthened image of the flame. This ex- 
periment may be performed without the slightest difficulty, and, 
as I have found, most easily, if the upper eyelid is retained in 
its position by the hand, and prevented from dropping down. 
If the white image has been obtained in about half a minute, 
-and the eyelid is allowed to drop, and the eye immediately re-° — 
opened, the spectrum appears for a moment with all its colours, 
and then changes rapidly into the white image. Here then we © 
have exactly the same effect produced by all the colours of the © 
spectrum, as was found above to be the case with the iodide of — 
silver. 
Brewster’s experiment is so striking a confirmation of the — 
theory in question, that we may well ask how the usual opinion 
concerning vision can explain it. Without doubt the comple- 
