M. ARAGO ON THE CHEMICAL ACTION OF LIGHT. 561 
“1 immediately hastened to make these experiments, begin- 
ning by using water as a new medium. My departure for the 
country compelled me to interrupt them. I expected to resume 
them at my return, before acquainting you with the result, but 
the badness of the season has not allowed me to fulfill my inten- 
tion; I have nevertheless the honour of sending you the result 
of two experiments which I have made, with the description of 
the process I have followed. 
“T made use of a small glass reservoir, the sides of which 
‘were very flat, and of a plate prepared according to M. Da- 
_guerre’s method, which could be placed vertically in the reser- 
voir so that its surface should be parallel with the front face of the 
reservoir. In these two experiments the distance between the 
iodized plate and this face was one centimetre. A pencil of 
solar rays is then introduced into a dark chamber through a 
_ narrow aperture made in the shutter; these rays are refracted 
_ through a prism of very pure rere ee before which is placed 
ee lens having a long focus, so as to obtain a solar spectrum by 
‘projection with all its lines. This result once obtained, I placed 
the reservoir before the path of the refracted ray, so that the 
_spectrum is depicted horizontally with all its lines on the iodized 
plate and so that the violet rays enter perpendicularly at the an- 
terior face of the reservoir. Before beginning the experiment care 
_ had been taken to pour water into the vessel until its surface cut 
the i image of the spectrum longitudinally into two equal parts. 
_ “Ifthe plate be taken away after one or two minutes of action, 
by exposing it to mercurial vapour the image of the spectrum 
is seen depicted from the limit of the green said of the blue to 
‘beyond the extreme violet, and, as I said in the Memoir, this 
image has all its lines similar to those of the luminous spectrum 
as regards the portions of the same refrangibility. Well! no 
| perceptible difference is to be seen between the i image of the 
' 
: 
spectrum on the portion of the plate which remained in the air 
and that which was formed on the portion which had been kept 
in the water; the lines of these two portions of spectrum seem 
very exactly to be prolongations of each other, excepting, 
however, in the extreme portions of the chemical spectrum to- 
wards the right and the left, where the lines of the image which 
was produced in the water seemed to become a little closer to 
each other. This it appears to me should be attributed to the 
refraction of the oblique rays. 
2pe2 
