CHRISTIAN HUYGHENS 6i 



nothing in the way of our imagining the particles of ether to have an 

 almost complete hardness, and an elasticity as perfect as we need wish. 

 We need not here discuss the cause of either this hardness or elastic- 

 ity, as this would lead us too far from the question at issue. I will 

 remark, however, by the way, that these particles of ether, in spite of 

 their minuteness, are also composed of parts and that their elasticity 

 depends on a very rapid motion of a subtle substance traversing them 

 in all directions and making them take a structure that offers a ready 

 passage to this fluid. This agrees with the idea of M. Descartes, ex- 

 cept that I would not, like him, give the pores the shape of round, 

 hollow canals. This is so far from being at all absurd or incompre- 

 hensible that it is easily credible that nature uses an infinite series of 

 different-sized molecules in order to produce her marvelous effects. 



Moreover, although we do not know the cause of elasticity, we 

 cannot have failed to notice that most bodies possess this character- 

 istic; hence it is not unreasonable to suppose that it is a quality of 

 the minute, invisible particles of the ether. And it is a fact that if 

 one looks for some other method of accounting for the gradual trans- 

 mission of light, he will have a hard time finding any supposition 

 better suited than elasticity to explain the fact of uniform speed. This 

 [uniform speed] seems to be a necessary assumption, for if the 

 motion slowed down when distributed over a great mass of matter 

 at a far distance from its source, then this great speed would at last 

 be lost. On the other hand, we suppose ether to have the property 

 of elasticity so that its particles regain their shape with equal activity 

 whether struck a hard or gentle blow. Thus the rate at which light 

 would move would remain constant. 



