556 E. BECQUEREL ON THE CONSTITUTION 
ing to such or such a sensible substance, the active rays we 
also different ; but I do not suppose that the question is so com- 
plex. In fact, the luminous phenomena, according to the theory 
of undulations, depend on the vibrations of the molecules of th 
illuminating body, which are transmitted to the retina by th 
intermediation of the zther, the molecules of which are them- 
selves in vibration. Fresnel, whose beautiful investigations have 
contributed to the triumph of this theory, had stated that the 
chemical effects produced by the influence of light are owing to 
a mechanical action exerted by the molecules of ether on the 
atoms of bodies, so as to cause them to assume new states o 
equilibrium dependent on the nature and on the velocity of the 
vibrations to which they are subjected. This idea had been 
suggested to him by a remarkable experiment of M. Arago’s, 
the result of which was to show that the chemical rays which 
influence the chloride of silver interfere in the same manner as 
the luminous rays. 
I think that the hypothesis of Fresnel is accurate, and even 
that itmay be extended further, especially if we consider that 
the chemical and phosphorogenic rays possess the same physical 
properties as the luminous rays; thus they are subjected to the 
physical laws of reflexion, of refraction, of double refraction, of 
polarization, and of interference, in the same manner as are these 
rays; and moreover, as we have seen in this memoir, the spectra 
of these different radiations have the same lines. Thus it would 
be more simple to suppose,— 
Ist. That a pencil of solar rays is the union of an infiniti 
number of rays of different refrangibility, each ray arising from 
undulations of zther not having the same velocity. 
2nd. That by refracting a pencil of solar rays through a prism, 
we have the solar spectrum which possesses different properties 
on account of its different action on external bodies. 
3rd. That with respect to certain substances the molecules of 
which are united by weak affinities, such as salts of silver, of 
gold, of mercury, &c., the solar rays act according to the velo- 
cities of undulation which may be transmitted to the molecules 
of matter, and consequently between certain limits of refrangi- 
bility. I have called the whole of the rays which affect a sub- 
stance a chemical spectrum. 
4th. That phosphorescent bodies becoming luminous by means 
of the molecular movement impressed on their molecules, a move-_ 
