LAW OF DISPERSION BY REFRACTION. 59 



rision is strongest in the outer circle, because the periphery is greater; 

 thus we shall have a gradation from red, through green, to purple, 

 in passing from the outer to the inner circle." This account would 

 hardly have deserved much notice, if it had not been for a strange 

 attempt to revive it, or something very like it, in modern times. The 

 same doctrine is found in the work of De Dominis. 4 According to 

 him, light is white : but if we mix with the light something dark, the 

 colors arise, first red, then green, then blue or violet. He applies 

 this to explain the colors of the rainbow, 5 by means of the considera- 

 tion that, of the rays which come to the eye from the globes of water, 

 some go through a larger thickness of the globe than others, whence 

 he obtains the gradation of colors just described. 



Descartes came far nearer the true philosophy of the iridal colors, 

 He found that a similar series of colors was produced by refraction of 

 light bounded by shade, through a prism ; 6 and he rightly inferred 

 that neither the curvature of the surface of the drops of water, nor the 

 reflection, nor the repetition of refraction, were necessary to the 

 generation of such colors. In further examining the course of the 

 rays, he approaches very near to the true conception of the case ; and 

 we are led to believe that he niiglit have anticipated Newton in his 

 discovery of the unequal refrangibility of different colors, if it had been 

 possible for him to reason any otherwise than in the terms and notions 

 of his preconceived hypotheses. The conclusion which he draws is, 7 

 that " the particles of the subtile matter which transmit the action of 

 light, endeavor to rotate with so great a force and impetus, that they 

 cannot move in a straight line (whence comes refraction) : and that 

 those particles which endeavor to revolve much more strongly produce 

 a red color, those which endeavor to move only a little more strongly 

 produce yellow." Here we have a clear perception that colors and 

 unequal refraction are connected, though the cause of refraction is 

 expressed by a gratuitous hypothesis. And we may add, that he 

 applies this notion rightly, so far as he explains himself/ to account 

 for the colors of the rainbow. 



It appears to me that Xewton and others have done Descartes 

 injustice, in ascribing to De Dominis the true theory of the rainbow. 

 There are two main points of this theory namely, the showing that a 

 bright circular band, of a certain definite diameter, arises from the 



4 Cap. iii. p. 9. See also Gothe, Farbenl. vol. ii. p. 251. 5 Gothe, p. 263. 

 6 Meteor. Sect. viii. p. 190. T Sect. vii. p. 192. s Meteor. Sect. ix. 



