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 is ground for using these three qualities of luminosity, transparence, 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 matter. 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 clement, are constantly straining away from the centres around which they turn, owing to the centrifugal force of the vortices; 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. In the Dioptrique, 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 be due to the different ways in which the matter moves. In the Météores, the various colours are connected with different rotatory velocities of the globules, the particles which 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