440 PROFESSOR LUDWIG MOSER ON VISION, 
were; it closes, and it is only after a greater or less space of 
time that we are enabled to see by this light. On the other 
hand, the greater degree of sensibility for light of low intensity 
is acquired only after a certain period, as is well known. I have | 
already mentioned, when describing Brewster’s experiment, that 
when, by continued observation, the colours of the spectrum 
have been converted into white, they appear again immediately 
for an instant, if the upper eyelid be allowed to drop. It is 
possible that this motion produces a certain degree of pressure, 
and this generally causes a displacement of the ocular lens. 
We involuntarily press an eye that has been strongly dazzled ; 
if we have an after image on the retina which is on the point of 
vanishing, a pressure on the eye, produced by the external mus- 
cles, generally serves to reproduce it. The pressure of the 
fingers produces luminous appearances; the stronger effort, as 
is well known, a fiery circle, the less degrees colours; and, as 
Prof. J. Miiller states, it is possible to change one colour into the 
other by varying the degree of force. The usual description is 
not sufficient for these phenomena, at least it does not offer an 
explanation of them; for it might then be asked why pressure 
does. not produce analogous effects on the organs of smell and 
taste, not to mention those of hearing, in which something takes 
place similar to the actions of the retina, but which allows of a 
different explanation. 
In adapting the eye to different intensities of light, the pupil 
itself certainly takes a part; but still only ina small degree. Its 
changes take place within much too narrow limits for it to be 
able to regulate the comparatively large variations of intensity. 
And, moreover, the pupil ‘alters when the intensity of light re- 
mains precisely the same. When Olbers*, for instance, measured 
the diameter of his own pupils in a mirror, he found it to be 
2:01" at a distance of 4", and 
2°74" at a distance of 28". 
The opening of the pupil had increased, therefore, in the pro- 
portion of 1°86, although the observed object retained the same 
intensity of light. According to Lambert the changes are much 
greater t. It has been proved that the expansion and con- 
traction of the eye are in general destined for other purposes than 
* G. R. Treviranus, Anatomie und Physiologie der Sinneswerkzeuge. Bre- 
men,1828, Heft |. 
+ Photometria, &c. § 855. 
