in the Seventeenth Century. 5 



is ground for using these three qualities of luminosity, trans- 

 parence, and opacity, in order to distinguish the three elements 

 of the visible world.* 



According to Descartes' theory, the sun is the centre of an 

 immense vortex formed of the first or subtlest kind of inatter.f 

 The vehicle of light in interplanetary space is matter of the 

 second kind or element, composed of a closely packed assemblage 

 of globules whose size is intermediate between that of the 

 vortex-matter and that of ponderable matter. The globules of 

 the second element, and all the matter of the first element, are 

 constantly straining away from the centres around which they 

 turn, owing to the centrifugal force of the vortices ;J so that the 

 globules are pressed in contact with each other, and tend to 

 move outwards, although they do not actually so move. It is 

 the transmission of this pressure which constitutes light ; the 

 action of light therefore extends on all sides round the sun and 

 fixed stars, and travels instantaneously to any distance. |j In 

 the Dwptrique$ vision is compared to the perception of the 

 presence of objects which a blind man obtains by the use of his 

 stick ; the transmission of pressure along the stick from the 

 object to the hand being analogous to the transmission of 

 pressure from a luminous object to the eye by the second kind 

 of matter. 



Descartes supposed the " diversities of colour and light " to 

 he due to the different ways in which the matter moves.** In 

 the Meteores,^ the various colours are connected with different 

 rotatory velocities of the globules, the particles winch rotate most 

 rapidly giving the sensation of red, the slower ones of yellow, and 

 the slowest of green and blue the order of colours being taken 

 from the rainbow. The assertion of the dependence of colour 



* Principia, Part iii, 52. 



t It is curious to speculate on the impression which would have been produced 

 had the spirality of nehulse heen discovered hefore the overthrow of the Cartesian 

 theory of vortices. 



J Ibid., 55-59. Ibid., 63. || Ibid., 64. IT Discours premier. 



** Principia, Part iv, 195. ft Discours Huitieme. 



