4 BIOLOGICAL EFFECTS OF RADIATION 



Herein h stands for a constant which must have the same value in both 

 equations; c for the speed of the waves; and v for the quotient of wave 

 speed by wave-length, which is by definition the frequency of the waves. 

 Nothing in the theory prescribes the values of h and c. The value of the 

 former is found, by experiments of many kinds (some of which I shall 

 later describe), to be this: 



h = 6.55 • 10-" erg/sec. 



with an uncertainty of less than two units in the third significant figure. 

 It is known as Planck's constant. The value of c is found, by experiments 

 on the velocity of light signals^ in air or vacuum, to be so close to 3 • 10^" 



that this simple figure is quite accurate enough for most purposes. 

 Actually the value in vacuum (to which alone the symbol c should in 

 strictness be applied) is given as 2.99796 + 0.00004 cm./sec. (times lO^"). 



The derivation of equation {1) is so interesting in itself, and in its 

 relation to a famous controversy of a century ago, that it is worthy of 

 retracing here. We shall interpret one of the simplest of optical phe- 

 nomena — the refraction of a ray of light at a surface of discontinuity 

 between two contrasting media, air and water, for example — by the wave 

 picture and the corpuscle picture in succession. 



The reasoning from the wave theory is usually made in graphic fashion 

 by showing ''Huygens' construction" (Fig. 1) which should remind many 

 a reader of his high-school days! This is a very crude form of wave 



1 There is a peculiar and difficult point in this connection. What is measured in 

 such experiments is not the wave speed but a thing entirely different in principle, the 

 so-called "group speed." The two are very different in transparent solid or liquid 

 media such as carbon bisulphide, but in a medium in which wave speed does not vary 

 with wave-length they should be the same, and such appears to be the case in the 

 medium which we know as empty space. I have developed this point at length in 

 pp. 28-30 of an article "Elementary Notions of Quantum Mechanics" in Reviews of 

 Modern Physics, 6, January, 1934. 



