66o THE POPULAR SCIENCE MONTHLY. 



corded many observations and measurements on the subject of refrac- 

 tion. To them succeeded Kepler, who, taking the results tabulated 

 by his predecessors, applied his amazing industry to extract from them 

 their meaning — that is to say, to discover the physical principles which 

 lay at their root. In this attempt he was less successful than in his 

 astronomical labors. In 1604 Kepler published his " Supplement to 

 Vitellio," in which he virtually acknowledged his defeat, by enunciat- 

 ing an approximate rule, instead of an all-satisfying natural law. The 

 discovery of such a law, which constitutes one of the chief corner- 

 stones of optical science, was made by Willebrord Snell, about 1621.* 



A ray of light may, for our purposes, be presented to the mind as 

 a luminous straight line. Let such a ray be supposed to fall vertically 

 upon a perfectly calm water-surface. The incidence, as it is called, is 

 then perpendicular, and the ray goes through the water without devia- 

 tion to the right or left. In other words, the ray in the air and the 

 ray 'in the water form one continuous straight line. But the least 

 deviation from the perpendicular causes the ray to be broken, or " re- 

 fracted," at the point of incidence. What, then, is the law of refrac- 

 tion discovered by Snell ? It is this, that no matter how the angle of 

 incidence, and with it the angle of refraction, may vary, the relative 

 magnitude of two lines, dependent on these angles, and called their 

 sines, remains, for the same medium, perfectly unchanged. Measure, 

 in other words, for various angles, each of these two lines with a scale, 

 and divide the length of the longer one by that of the shorter ; then, 

 however the lines individually vary in length, the quotient 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 co- 

 herence to the scientific organism. Were it not for the antecedent 

 discovery of the law of sines, founded as it was on exact measure- 

 ments, 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 inclosed 

 an angle of 41°. Whence this steadfastness of position — this inflexi- 

 ble adherence to a particular angle ? Newton gave to De Dominis f 

 the credit of the answer ; but we really owe it to the genius of Des- 

 cartes. He followed with his mind's eye the rays of light impinging 

 on a rain-drop. He saw them in part reflected from the outside surface 

 of tlie drop. He saw them refracted on entering the drop, reflected 



* Born at Leyden 1591 ; died 1626. 



f 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 (Poggen- 

 dorfPs "Biographical Dictionary "J, 



