458 PROFESSOR LUDWIG MOSER.ON VISION, 
the modification is destroyed as easily as it is produced. Expe- 
riments have shown that it disappears of itself without the in- 
terference of external causes; Draper found that iodized plates 
which only required exposure to the vapours of mercury to pro- 
duce the images, if allowed to remain untouched several days, 
did not afterwards exhibit a trace of the picture when treated 
with mercury, although the iodide had not changed, and was still 
perfectly sensitive with regard to light. We must not overlook 
the fact to which I have already directed attention, viz. that the 
modifications which we produce on the plates of silver by means 
of light in order subsequently to form a picture, are considerably 
less than those necessary for vision to be enabled to act. It is 
probable that a very short period of repose would be sufficient 
to destroy this small degree of modification. If, however, in the 
case of vision, we allow the action to proceed further, by regard- 
ing for a length of time a strongly illuminated object, then the 
retina naturally does not so soon regain its normal state, and 
even lasting ill consequences may ensue. The images discovered 
by Riess by means of electric action, and which were visible 
after a lapse of more than four years, were owing to the higher 
degrees of modification. Disregarding these, we find that the 
first stages are easily destroyed, by a slight friction, by the 
breath, &c. Very inconsiderable causes act on bodies, for I have 
found that if a polished plate of silver be allowed to remain un- 
touched a few minutes, the process of iodizing requires a much 
longer time than if the plate had been immediately exposed to 
the vapours of iodine. This cannot be explained so easily ; for 
light requires a much longer time to exert its influence on these 
plates than on others, although they are iodized just the same. 
We see then by what insignificant causes the modified state 
of the surface is changed or destroyed, and we therefore under- 
stand why the system of after-images, although always present 
to the eye, is still not of any very great importance. The pres- 
sure of the outer muscles of the eye, whose action extends to 
the retina, is here of great advantage, as is also the continual 
motion of the eye itself, which produces successively the most 
varied states of the separate parts of the nervous membrane. 
Moreover, there is a point which must not be forgotten at the 
end of this treatise, but to which we have not as yet alluded, in 
order not to put a stop to all further research, viz. that the retina 
is an organic formation, and that the substance of the nerves can 
be so easily restored. 
