AND ITS CONGENERS. 201 



yielded by this division remains absolutely the same. 

 It is, in fact, what is called ' the index of refraction ' of 

 the medium. 



Science is an organic growth, and accurate 

 measurements give coherence to the scientific organism. 

 Were it not for the antecedent discovery of the law of 

 sines, founded as it was on exact measurements, the 

 rainbow could not have been explained. Again and 

 again, moreover, the angular distance of the rainbow 

 from the sun had been determined and found constant. 

 In this divine remembrancer there was no variableness. 

 A line drawn from the sun to the rainbow, and another 

 drawn from the rainbow to the observer's eye, always 

 enclosed an angle of 41°. Whence this steadfastness 

 of position — this inflexible adherence to a particular 

 angle ? Newton gave to De Dominis l the credit of the 

 answer ; but we really owe it to the genius of Descartes. 

 He followed with his mind's eye the rays of light im- 

 pinging on a raindrop. He saw them in part reflected 

 from the outside surface of the drop. He saw them re- 

 fracted on entering the drop, reflected from its back, 

 and again refracted on their emergence. Descartes 

 was acquainted with the law of Snell, and taking up 

 his pen he calculated, by means of that law, the whole 

 course of the rays. He proved that the vast majority 

 of them escaped from the drop as divergent rays, and, 

 on this account, soon became so enfeebled as to produce 

 no sensible effect upon the eye of an observer. At one 

 particular angle, however — namely, the angle 41° afore- 

 said — they emerged in a practically parallel sheaf. In 

 their union was strength, for it was this particular 



1 Archbishop of Spalatro and Primate of Dalmatia. Fled to 

 England about 1616 ; became a Protestant, and was made Dean of 

 Windsor. Returned to Italy, and resumed his Catholicism ; but was 

 handed over to the Inquisition, and died in prison (Poggendorff 's 

 Biographical Dictionary} 



