I0 4 SEVENTEENTH CENTURY. PT. in. 



they would not have denied the truth of Galileo's experiment 

 when it succeeded 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 

 conjtradict 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. 



Thus 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. 



Sncllius Discovers the Law of Refraction, 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 be- 

 fore, in 1621, 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 

 angles at which rays are refracted in water and glass, but 

 they did not know of any law by which they could calculate 

 how much any particular ray would be bent out of its course. 



For instance, in Fig. 12, suppose w w to be the surface 

 of water in a glass vessel, upon which the rays A and B fall 

 at the point o, and are refracted A to A' and B to B'. It is 

 evident that B is bent much more out of its course than A, 



