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 conciudes 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 
of 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==/ sin r), a constant ratio between the sines, not 
the angles, where 7 and r are the angles of incidence and refraction, 
and mw 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 pudlish any 
account of the matter till it appeared in his Dioptrica, which was 
- printed after his death in 1700. 
