456 Professor Tyndall [Jan. 18, 



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

 a luminous straight line. Let sucli 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 

 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 perpendicular causes the ray to be broken, or 

 " refracted," at the point of incidence. What, then, is the law of 

 refraction 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 

 coherence to the scientific organism. Wore 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 drawTi 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 Domini s * 

 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 raindrop. He saw them in part reflected from the outside surface 

 of the drop. He saw them refracted 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 cal- 

 culated, 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 jjar- 

 ticular angle, however — namely, the angle 41° aforesaid — they 

 emerged in a practically parallel sheaf. In their union was strength, 

 for it was this particular sheaf which carried the light of the 

 " primary " rainbow to the eye. 



There is a certain form of emotion called intellectual pleasure, 

 which may be excited by poetry, literature, nature, or art. But I 



♦ 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 (PoggendorflTs ' Biographical Dictionary '). 



