1 86 THE QUANTUM THEORY 



energy endowed with a certain period, and the product 

 of the two is //. The period is carried by the waves 

 without change, but the energy spreads out in an ever- 

 widening circle. Eight years and nine months after the 

 emission the wave-front is due to reach the earth. A 

 few minutes before the arrival some person takes it into 

 his head to go out and admire the glories of the heavens 

 and — in short — to stick his eye in the way. The light- 

 waves when they started could have had no notion what 

 they were going to hit; for all they knew they were 

 bound on a journey through endless space, as most of 

 their colleagues were. Their energy would seem to be 

 dissipated beyond recovery over a sphere of 50 billion 

 miles' radius. And yet if that energy is ever to enter 

 matter again, if it is to work those chemical changes in 

 the retina which give rise to the sensation of light, it 

 must enter as a single quantum of action h. Just 

 6-55. 1 o -27 erg-seconds must enter or none at all. Just 

 as the emitting atom regardless of all laws of classical 

 physics is determined that whatever goes out of it shall 

 be just /*, so the receiving atom is determined that what- 

 ever comes into it shall be just h. Not all the light- 

 waves pass by without entering the eye; for somehow 

 we are able to see Sirius. How is it managed? Do the 

 ripples striking the eye send a message round to the 

 back part of the wave, saying, "We have found an eye. 

 Let's all crowd into it!" 



Attempts to account for this phenomenon follow two 

 main devices which we may describe as the "collection- 

 box" theory and the "sweepstake" theory, respectively. 

 Making no effort to translate them into scientific 

 language, they amount to this: In the first the atom 

 holds a collection-box into which each arriving group 

 of waves pays a very small contribution; when the 



