200 THE RAINBOW 



made and recorded many observations and measurements 

 on the subject of refraction. 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 prin- 

 ciples which lay at their root. In this attempt he was 

 less successful than in his astronomical labours. In 

 1604 Kepler published his ' Supplement to Vitellio,' in 

 which he virtually acknowledged his defeat by enun- 

 ciating an approximate rule, instead of an all-satisfying 

 natural law. The discovery of such a law, which con- 

 stitutes one of the chief corner-stones of optical science, 

 was made by Willebrord Snell, about 162 1. 1 



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 perpen- 

 dicular, and the ray goes through the water without 

 deviation 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 perpen- 

 dicular causes the ray to be broken, or c refracted,' at 

 the point of incidence. What, then, is the law of re- 

 fraction discovered by Snell ? It is this, that no matter 

 how the angle of incidence, and with it the angle of re- 

 fraction, may vary, the relative magnitude of two lines, 

 dependent on these angles, and called their sines, re- 

 mains, 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 



as a German, born in Thiiringen. ' Vitellio ' is described as a 

 corruption of Witelo. 



1 Born at Leyden, 1591; died 1626. 



