AND ON THE ACTION OF LIGHT ON BODIES. 457 
We cannot but suppose that the modifications of the bodies 
are accompanied by other physical and chemical changes beyond 
those already observed ; hyposulphite of soda has already afforded 
an instance. It dissolves the variously modified iodide of silver 
with different degrees of ease, and the blackened iodide not at 
all; consequently the behaviour of some of these modifications 
is chemically different. 
There are many other suppositions which occur to me; I will 
only mention one, viz. that it is possible the modifications stand 
in close connexion with the hitherto so mysterious phenomena 
of phosphorescence. A step towards its explanation has been 
made by the discovery of Riess*, that polished plates even of 
conducting substances, which were situated along the path of 
the electrical explosion, or upon which the electricity has passed 
in the form of brushes of light, afterwards exhibit spots which 
do not condense aqueous vapours, and which are evidently in 
that state which we have produced in such different methods. 
Moreover, it is known that when the electric spark passes over 
a suitable substance, it renders it phosphorescent, and that in 
this case coloured glasses have the same influence as in the so- 
ealled chemical actions of light. It even seems as if phospho- 
rescence had been produced by contact alone, although the value 
of this agent has not been noticed}. But these points must be 
reserved for future experiments. 
I now return to the original object of this memoir, viz. the 
Action of Light on the Retina. I am inclined to believe that 
the state of this postulate has considerably changed, and that it 
will no longer appear as a daring hypothesis. If light produce 
the same modification in all bodies, it will do so in the retina; 
nothing is more natural. The questions, how the retina alters 
its sensibility, and how it regains its normal state, have been 
answered approximately, although not definitely. The modi- 
fications produced by light are of themselves subject to dif- 
ferent variations, and can be produced by the same intensity of 
light if different times be employed. The preceding experiments 
have shown that there are means of making an iodized plate 
more or less sensitive without the assistance of the vapours of 
chloride or bromide of iodine, for instance, by means of contact 
* Poggendorff’s Annalen, Bd. xliii. s. 85. Repertorium der Physik, Ba. vi. 
s. 180. 
+ Poggendorff’s Annalen, Bd. xlvi. p. 613. 
VOL. III. PART XI. 2H 
