562 M. ARAGO ON THE CHEMICAL ACTION OF LIGHT. 
“ These two experiments tend to show that the nature of the 
medium into which the substance chemically sensitive to the 
action of the solar rays is plunged, does not modify the action of 
these, so that the impression of the solar spectrum on this sub- 
stance always presents the same lines and at the same places. 
“ As soon as I have time I intend to make these experiments 
again, to vary them, and perhaps I shall come to more conclu- 
sive results. JI have the honour to be,” &c. 
Here then are the solar rays acting exactly in the same way 
in air and in water. In air, however, according to the system 
of emission, light moves much less quickly than in water. The 
velocity here, therefore, is without influence, a result which at 
first sight seems in manifest contradiction to what we deduced 
from the first experiment. The two results however may be 
reconciled. A new hypothesis may, it seems to me, make them 
agree; but every one will form his own judgement about them. 
The velocity with which a luminous ray passes through a 
given body depends exclusively on the refringency of this body 
and on the velocity of emission of the ray, on the velocity it had 
in vacuo. The ray which reached the surface of the stratum of 
iodine through the water at the point where it meets this surface, 
possesses a velocity superior to that which the ray that moved 
through the air had at the same point; but in the interior itself 
of the stratum, at a sufficient depth, the two rays possess exactly 
the same velocity. Let us make the photogenic phenomena 
depend not upon an action exerted at the surface, but upon an 
action originating in the interior of the stratum, and every dif- 
ficulty disappears ; only,—a singular result,—we are compelled 
to establish an essential distinction between the interior and the 
surface of a stratum, the thickness of which is incredibly small, 
By thus considering the photogenic phenomena as examples 
of molecular actions susceptible of exact estimation, every one 
will feel how interesting it would be to intercalate figures in the 
general arguments which I have just offered. We shall attain 
this object by first completing the experiments by help of which 
M. Dumas had begun to determine the thickness of the stratum 
of iodine on which the Daguerrian images are formed from the 
comparative weight of a large silvered plate before and after its 
iodation. Afterwards as much accuracy as possible will be used 
in observing the relative positions of the dark lines traced on 
