438 ‘PROFESSOR LUDWIG MOSER ON VISION, 
compared with that of other known sensitive bodies. This sensi- 
bility may be regarded in two points of view :—I1st, with regard 
to its degree, and 2ndly, as to its variability; for the eye un- 
questionably possesses the power of adapting its sensibility to 
the intensity of the light, and it must also possess means of re- 
storing this sensibility after it has been taken away. I intend 
to show that in both cases there is nothing in which the retina 
differs particularly from other substances subjected to the influ- 
ence of light. 
In the first place, as regards the degree of sensitiveness, it 
certainly is extremely great in the case of the retina; but we need 
not consider it immeasurably greater than it is in the case of the 
silver compounds. It has been already stated that the exposure 
of the iodide to the vapours of chloride of iodine increases its 
sensibility very greatly (according to my experiments the time 
appeared to be reduced to one-tenth of what was otherwise re- 
quisite). Gaudin * has lately said that the vapours of bromide 
of iodine may be still more advantageously employed; and he 
states that by its means he has obtained images in the space of 
one-tenth of a second. We must not however forget, that in 
order to procure an image according to Daguerre, we make a 
certain demand on the light, viz. to bring the iodide of silver 
into such a state that it will condense the vapours of mercury. 
This is a special demand, which, if it is to be fulfilled, requires 
that the plate should remain a certain time in the camera ob- 
scura. The light, however, has acted much earlier, which could 
be proved if we possessed reagents efficient for the purpose. 
Red glass is one of these ; it shows us that long before the plate 
is usually taken out of the camera obscura the image existed 
with all its shades and details ; and without doubt we shall here- 
after discover other means by which we may prove the existence 
of the image in its earlier stages of development. 
The period at which we make use of the action of the light 
cannot in reality be compared to the time in which external 
objects become visible to the retina. It is analogous to that 
state of the retina in Brewster’s experiment, when the colours 
of the spectrum disappear, and there only remains a uniform 
impression of white. ‘This is an unnatural state, which can only 
be attained after continued observation for a half or whole mi- 
nute. If, therefore, we wish to make a comparison between the 
* Comptes Rendus, iebd, 18 Octobre, 1841. 
