ACTION OF LIGHT ON CERTAIN BODIES. 245 
deviation is the necessary consequence of this supposi- 
tion, since in the multitude of rays which strike on the 
eye, whether it is apparently towards or receding from 
the stars, it will perceive, in either case, those only whose 
molecules have the same relative velocity; but this 
hypothesis, it cannot be denied, deprives the system of 
emission of the simplicity which constitutes its main 
recommendation. The clashing of molecules on which 
Euler so much insisted, would then become the inevita- 
ble consequence of their inequality of velocity, and would 
entail on the propagation of the rays disturbances to 
which observation does not show them to be subject. 
Light exercises a striking action on certain bodies ; it 
rapidly changes their colour. Nitrate of silver, as is well 
known, possesses for example this power in a high 
degree. It suffices to expose it for a few seconds to the 
diffuse light of a cloudy sky for it to lose its original 
whiteness, and to become of a bluish black. In the rays 
of the sun it changes almost instantaneously. Chemists 
have believed that they could see in this discoloration a 
phenomenon analogous to that they produce every day. 
According to them the light would be a true “reagent,” 
which in being added to the constituent principles of the 
compound on which it acts, sometimes modifies its origi- 
nal properties ; sometimes also the luminous matter only 
determines by its action the disengagement of one or 
more elements of the body on which it strikes. 
These explanations, although based on specious analo- 
gies, do not seem to be admissible, since it has been 
shown that, in interfering, the luminous rays also lose 
the chemical properties with which they are endowed. 
How can we conceive, in fact, that the matter of two 
rays can combine with a given substance if each ray 
