K)6 SEVENTEENTH CENTURY. rx. in. 



we have never tried, but we think it probable Aristotle was 

 right until someone shows us that he was mistaken ; ' — if they 

 had gone to the Tower of Pisa in this spirit, they would not 

 have denied the truth of Galileo's experiment when it suc- 

 ceeded before their very eyes. And even now, in the 

 present day, you will see that the greatest and best men who 

 make the most discoveries, are those who are always willing 

 to examine a new fact, even though it may contradict much 

 that they have held before ; and who never pretend to know 

 for certain anything which they have not studied with 

 sufficient care to be convinced of its truth. 



These last few pages may be rather difficult for you to 

 follow, but the chief lessons which it is necessary you should 

 remember may be summed up in a few words. Bacon and 

 Descartes both did great service to Science — Bacon by 

 teaching that any true theory must be built up upon facts 

 and careful experiments ; Descartes by insisting that it is 

 more honest to acknowledge we are ignorant, and to wait 

 for more light, than to pretend to know that which we have 

 not clearly proved. 



Snellius Discovers the Law of Eefraction, 1621. — 

 Among other things, Descartes wrote much upon Optics, and 

 you will often see it stated that he discovered the law of 

 refraction. This law had, however, been laid down before, 

 in 162 1, by a Dutch mathematician named Willebrord 

 Snellius, and Descartes only stated it more clearly. You 

 will remember that the Arab Alhazen first pointed out that 

 rays of light are bent or refracted when they pass from a 

 rarer into a denser substance or medium (see p. 47), as for 

 instance from air into water; and that the denser the 

 medium is into which they pass the more the rays are re- 

 fracted. Vitellio and Kepler had measured some of the 



