AND ON THE ACTION OF LIGHT ON BODIES. 439 
iodide of silver and the retina, in respect to the necessary dura- 
tion of the light’s influence, then the time which the plates re- 
quire to remain in the camera obscura would be analogous to the 
time required by the retina to pass into the above-mentioned 
anomalous state; and these periods are commensurable. One 
of my acquaintances obtained the perfect image of a common 
candle-flame on pure iodide of silver in the space of two minutes, 
Draper that of an Argand gas flame in fifteen seconds, and with- 
out chloride of iodine. These are by no means much greater 
spaces of time than the retina requires to assume a similar state. 
These periods will certainly become greater when the intensity 
of the light is diminished; but there a comparison with the retina 
is not possible, because it cannot be brought into that anoma- 
lous state at all, if the colours do not possess a considerable 
degree of intensity. 
It is the general opinion that ordinary vision does not take 
place instantaneously, but that it requires a definite though cer- 
tainly very minute space of time to bring it into action. This 
is of itself very probable; and Fechner concludes that the 
strongest impresssion on the eye only takes place after the lapse 
of a certain time, from the fact that a black spot on a rapidly 
revolving white disc leaves behind it a dark circular track, and 
also a white spot on a black disc leaves a light mark*. The 
same philosopher explains an analogous phenomenon visible 
with coloured discs in a precisely similar manner +. It is just 
the same with the retina as with the sensitive papers, which are 
affected very rapidly at first, as I and others have found, indeed 
quicker than the iodide of silver, but which require a long time 
before they exhibit the after phenomena which I have described 
above. This is also the case with the retina, inasmuch as it 
produces the after images analogous, as I shall show, to those 
later appearances, only after the light has acted for a length of 
time. 
Concerning the second point, viz. the variability of sensi- 
tiveness, there is no doubt that the eye is capable of producing 
it, although, according to the usual theory of vision, when the 
oscillations were traced to the retina, and the consideration of 
their action then dropped, this point has never become the 
subject of discussion. A strong light at first stuns the eye, as it 
* Poggendorff’s Annalen, Ba. 1. S. 202. 
+ Fechner’s Repertorium, Bd. ii. S. 213. 
