The Seventeenth Century 97 



regarded light as rapid motion in a subtle (elastic) fluid (the 

 aether) , made up of particles (the first element) invading all space, 

 even the pores of solid bodies like glass. Space was a plenum, with 

 no vacuum anywhere. Its particles moved in vortices, not in straight 

 lines. Light was not a transmission of particles but was communi- 

 cated by the push or pressure of one particle to another, as the 

 motion of one end of a stick is felt at the other. The sun's par- 

 ticles were in agitation and transmitted this agitation instantane- 

 ously ^' throughout the Universe. 



It will be noted that Descartes' point of view clearly distinguished 

 between the manner in which light was generated, by agitation and 

 friction of particles, and the manner in which it was transmitted, 

 by the motion of a subtle liquid communicated by the pressure of 

 the moving body. 



Descartes took it for granted that the eyes of cats had the power 

 of emitting light, and, like others of his time, thought some men 

 might see better because of the light in their eyes. He presented 

 views on phosphorescence of the sea in 1637 in Les Meteores (see 

 Chap. XV) and later in his Philosophia, including also luminous 

 wood and fish. His explanation is best given by quoting the para- 

 graph in Principia Philosophia (Amsterdam, 1644; Part IV, Sec. 91) 

 which answers the question, " What is the light of the sea, rotten 

 wood, etc.? " 



But about what concerns sea water— the nature of which I have ex- 

 plained above, it is easy to see that the light which appears around the 

 drops [of sea water] when they are distrubed by storms comes only from 

 the fact that this stirring up, while leaving the parts that are soft and 

 pliant joined together, makes the little points of the others which are 

 stiff and straight, like little darts, stand out and push with impetuosity 

 into the parts of the second element which they encounter. I believe 

 also that rotten wood, salt fish and other such bodies do not light except 

 at a time when an alteration in them restricts many of their pores so 

 that they contain only matter of the first element: whether this altera- 

 tion comes when some of their parts approach each other, while others 

 separate, as appears to occur in rotten wood; or when some other bodies 

 mix with them, as occurs with salt fish, which do not light except during 

 the times that the particles of salt enter their pores. 



In either case Descartes held that the friction between the parts 

 results in the appearance of light. 



Descartes had many followers, who expounded and elaborated 

 his views. Among such men as Francois Bayle (1622-1709) , Pere 

 Nicholas Malebranche (1638-1715) , and Antoine LeGrand (fl. 1650- 



1* Ole Roemer demonstrated a finite velocity of light in 1675. 



