LAW OF REFRACTION. 187 



glass, or whether they enter glass on passing out of 

 water. As to the mathematical law of these deviations, 

 which the Arabian Alhasen, the Pole Vitellio, Kepler, 

 and other physicists had sought in vain, it is to Descartes 

 that we owe its announcement : I say Descartes, and 

 Descartes * alone ; for if the later claims put forth by 



* In thus strongly claiming for Descartes the discovery of the law 

 of refraction which English writers ascribe to Willebrod Snell, Arago 

 might be supposed actuated by a feeling of national pride, which not, 

 unfrequently, perhaps, influenced him on questions of this kind. The 

 strong expression with which he concludes the sentence, seems, how- 

 ever, to point to a more philosophical motive, and to refer the claim 

 of Descartes to considerations derived from the connection of the law 

 o/ refraction with his theories. However this may be, it may be well 

 briefly to recapitulate the facts of the case. The ancients, especially 

 Ptolemy, had amassed many measured results. Alhasen (A.D. 1100) 

 stated the general principle that refraction in a denser medium causes 

 the ray to deviate nearer to the perpendicular. Vitellio collected a 

 number of measured results in different media at different angles of 

 incidence ; among which Kepler attempted, with his usual ardour, to 

 endeavour to deduce some general numerical relation. He, however, 

 could proceed no further than this that while the angle of incidence 

 is but small, it is in a constant ratio (dependent on the nature of the 

 medium; to that of refraction; but that, as we deviate more from the 

 perpendicular, the rule becomes less accurate, and soon fails. 



Willebrod Snell, in 1621, investigated and established, by com- 

 parison of numerical results, a general geometrical mode of repre- 

 senting the case, which, expressed in modern terms, is the true law of 

 refraction (or sin i=(J. sin r), a constant ratio between the sines, not 

 the angles, where i and r are the angles of incidence and refraction, 

 and (J. the constant or refractive index. And the relation observed 

 by Kepler, which is true so long as the angle is small enough to be 

 nearly proportional in its sine, is thus extended and generalized. 

 Snell died in 1626 without having printed his discovery; but it had 

 been shown in MS. to many persons, especially to Huyghens, who 

 fully perceived its value and importance. And it is on his authority 

 that the discovery was properly assigned to Snell by Montucla, 

 Bossut, and other writers. Huyghens, however, did not publish any 

 account of the matter till it appeared in his Dioptrica^ which was 

 printed after his death in 1700. 



