THE LAW OF REFRACTION 403 



ignorant of trigonometry how to calculate the index of 

 refraction. 



Biot attempts to give the whole glory of the discovery to 

 his countryman Descartes, but the preponderance of opinion 

 is in favour of the view that Descartes' discovery was not 

 independent. Playfair states that the question is not one on 

 which we can ever attain certainty, but that we must be in- 

 fluenced by what we know of Descartes' general character and 

 behaviour. Now, Descartes was not generous in speaking of 

 the work of others. In dealing with the invention of the 

 telescope, for example, he does not mention the name of Galileo, 

 and in treating of the rainbow he makes no reference to Antonio 

 de Dominis. It is only natural that this should produce an 

 unfavourable impression, when we come to consider his claims 

 in connection with the law of refraction. 



So much for the statement of the law. When we consider 

 its physical interpretation, we are confronted with the alterna- 

 tives presented by the emission theory and the wave theory. 

 According to the emission theory the ray consisted of a jet of 

 particles. When these impinged on the surface of the denser 

 medium some were reflected like billiard-balls from the 

 cushion. The impact was perfectly elastic ; the component of 

 velocity perpendicular to the reflecting surface was reversed, 

 while the other component was unchanged. Hence the law of 

 reflection. To explain the law of refraction it was necessary 

 to assume that the particles were attracted, when they 

 approached the denser medium, by the molecules of the latter. 

 Hence the normal component of the velocit}'' was increased. 

 This gave the law of refraction correctly enough, but it made 

 the velocity greater in the denser medium, a result which is 

 contrary to experience. The case of total reflection is specially 

 interesting on the emission theory ; the particles approach the 

 surface of the glass from within in straight lines, pass out into 

 the air, become subject to the attraction of the glass, describe 

 parabolic orbits in the air, and return into the glass again. The 

 totally reflected ray is analogous to particles shot up a tube 

 through the earth into the air ; the latter describe parabolic 

 orbits under the influence of the earth's gravitational field, and 

 return to the earth again. Arguing on this basis, since the rays 

 apparently emerged into the air at total reflection, Newton 

 decided, that if a second glass surface were brought up parallel 

 to the first, and the air-space between gradually diminished so 

 as to make the second glass surface touch the top of the parabolic 

 orbits, then the ray would enter the second piece of glass. He 

 performed the experiment, and found his conclusion justified. 

 At total reflection some light entered the second piece of glass, 

 when the two glass surfaces were separated by a thin film of 



