56 HISTORY OF OPTICS. 



his Supplement to Vitello, published in 1604, Kepler attempts tc 

 reduce to a rule the measured quantities of refraction. The reader 

 who recollects what we have already narrated, the manner in which 

 Kepler attempted to reduce to law the astronomical observations of 

 Tycho, devising an almost endless variety of possible formulae, tracing 

 their consequences with undaunted industry, and relating, with a viva- 

 cious garrulity, his disappointments and his hopes, will not be sur- 

 prised to find that he proceeded in the same manner with regard to 

 the Tables of Observed Refractions. He tried a variety of constructions 

 by triangles, conic sections, &c., without being able to satisfy himself; 

 and he at last 1 is obliged to content himself with an approximate rule, 

 which makes the refraction partly proportional to the angle of inci- 

 dence, and partly, to the secant of that angle. In this way he satisfies 

 the observed refractions within a difference of less than half a degree 

 each way. When we consider how simple the law of refraction is, 

 (that the ratio of the sines of the angles of incidence and refraction is 

 constant for the same medium,) it appears strange that a person at- 

 tempting to discover it, and drawing triangles for the purpose, should 

 fail ; but this lot of missing what afterwards seems to have been ob- 

 vious, is a common one in the pursuit of truth. 



The person who did discover the Law of the Sines, was Willebrord 

 Sncll, about 1621 ; but the law was first published by Descartes, 

 who had seen Snell's papers. 2 Descartes does not acknowledge this 

 law to have been first detected by another ; and after his manner, in- 

 stead of establishing its reality by reference to experiment, he pretends 

 to prove a priori that it must be true, 3 comparing, for this purpose, 

 the particles of light to balls striking a substance which accelerates 

 them. 



[2nd Ed.] [Huyghens says of Snell's papers, "Quae et nos vidimus 

 aliquando, et Cartesium quoque vidisse accepiuius, et hinc fortasse 

 mensuram illam quaj in sinibus consistit elicuerit." Isaac Vossius, De 

 Lucis Naturd- et Proprietate, says that he also had seen this law in 

 Snell's unpublished optical Treatise. The same writer says, " Quod 

 itaque (Cartesius) habet, refractionum momenta non exigenda esse ad 

 angulos sed ad lineas, id tno Snellio, acceptum ferre debuisset, cujug 

 nomen more solito dissimulavit." " Cartesius got his law from Snell, 

 and in Ids usual way, concealed it." 



1 L. U. K. Life of Kepler, p. 115. 

 a Huyghens, Dioplrtca, p. 2. 3 Diont. p. 53. 



