188 DISSERTATION SECOND. [pabt i. 



practice, in the cases where the angles just mentioned 

 were very small. 



The discovery of the true law of refraction was the work 

 of Snellius, the same mathematician whose labours concern- 

 ing the figure of the earth were before mentioned. In or- 

 der to express this law, he supposed a perpendicular to the 

 refracting surface, at the point where the refraction is made, 

 and also another line parallel to this perpendicular at any 

 given distance from it. The refracted ray, as it proceeds, 

 will meet this parallel, and the incident ray is supposed to 

 be produced, till it do so likewise. Now, the general 

 truth which Snellius found to hold, whatever was the posi- 

 tion of the incident ray, is, that the segments of the re- 

 fracted ray and of the incident ray, intercepted by these 

 parallels, had always the same ratio to one another. If 

 either of the media were changed, that through which the 

 incident ray, or that through which the refracted ray 

 passed, this ratio would be changed, but while the 

 media remained the same, the ratio continued unaltera- 

 ble. It is seldom that a general truth is seen at first 

 under the most simple aspect : this law admits of being 

 more simply expressed, for, in the triangle formed by 

 the two segments of the rays, and by the parallel which 

 they intersect, the said segments have the same ratio with 

 the sines of the opposite angles, that is, with the sines of 

 the angles of incidence and refraction. The law, there- 

 fore, comes to this, that, in the refraction of light, by the 

 same medium, the sine of the angle of incidence has to the 

 sine of the angle of refraction always the same ratio. This 

 last simplification did not occur to Snellius ; it is the work 

 of Descartes, and was first given in his Dioptricks, in 

 1637, where no mention is made of Snellius, and the law of 

 refraction appears as the discovery of the author. This 

 naturally gave rise to heavy charges against the candour 

 and integrity of the French philosopher. The work of 



