MATHEMATICAL SCIENCES SEVENTEENTH CENT. 159 



motion to a convex piece of iron. He applies the disc of glass, pre- 

 viously cemented to a piece of cork, to the revolving tool, which is 

 represented on a large scale in Fig. 66, being' either convex as at E or 



FIG. 65. 



concave as at A. Water and emery powder of progressive degrees of 

 fineness are used until the glass has finally attained its required form 

 and polish. 



To Descartes we are indebted for a much more complete explanation 

 of the rainbow than that formerly given by De Dominis. In his trea- 

 tises on Meteors, Descartes traced the paths of the rays, and showed 

 why rainbows always appear under certain angles only. He attributes 

 the colours to refraction, and describes the manner in which a coloured 



