ACTION OP LIGHT ON CERTAIN BODIES. 245 



deviation is the necessary consequence of this supposi- 

 tion, since in the muhitutle of rays which strike on the 

 eye, wliether 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 w^ould 

 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 



