M. ARAGO ON THE CHEMICAL ACTION OF LIGHT. 563 
the sensitive matter, even by using the microscope if necessary ; 
lastly, instead of making a sudden transition from air to water, 
we shall compare the relative positions of the lines produced in 
two media slightly different in density or in refrangibility. Jn 
the system of emission the following consequences are rigorously 
deduced from the discussion into which I have just entered: 
If the photogenic effects of solar light result exclusively from 
the action of obscure rays mixed with the visible rays, proceed- 
ing like them and with velocities of the same order, the super- 
posed spectra of these two species of rays have their solutions 
of continuity exactly at the same places ; 
If the visible rays produce the photogenic effects totally or 
in part, this property is so inherent in their velocity that they 
equally lose when this velocity increases and when it diminishes ; 
The photogenic effects of solar light, whether they proceed 
_ from visible or invisible rays, cannot be attributed to an action 
exerted at the surface of the sensitive stratum ; it is in the in- 
_terior of the substance that the focus of this kind of action is to 
_ be sought for. 
_ The preceding conclusions may be extended when we know 
the thickness of the least stratum of iodine in which the Da- 
guerrian phenomena are produced, when it is possible to com- 
pare this thickness to the length of the fit or to that of the 
_ luminous waves. 
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