380 



HISTORY OF OPTICS. 



once adopted. In particular we must consider him 

 as the genuine author of the explanation of the 

 rainbow. It is true, that Fleischer 4 and Kepler 

 had previously ascribed this phenomenon to the rays 

 of sunlight which, falling on drops of rain, are 

 refracted into each drop, reflected at its inner sur- 

 face, and refracted out again : Antonio de Dominis 

 had found that a glass globe of water, when placed 

 in a particular position with respect to the eye, 

 exhibited bright colours ; and had hence explained 

 the circular form of the bow, which, indeed, Aris- 

 totle had done before 5 . But none of these writers 

 had shown why there was a narrow bright circle 

 of a definite diameter; for the drops which send 

 rays to the eye after two refractions and a reflec- 

 tion, occupy a much wider space in the heavens. 

 Descartes assigned the reason for this in the most 

 satisfactory manner 6 , by showing that the rays 

 which, after two refractions and a reflection, come 

 to the eye at an angle of about forty-one degrees 

 with their original direction, are far more dense 

 than those in any other position. He showed, in 

 the same manner, that the existence and position 

 of the secondary bow resulted from the same laws. 

 This is the complete and adequate account of the 

 state of things, so far as the brightness of the bows 

 only is concerned ; the explanation of the colours 

 belongs to the next article of our survey. 



4 Mont. i. 701. * Meteorol. iii. 3. 



6 Meteorum, cap. viii. p. 196. 



