378 SCIENCE PROGRESS 



In the same way, if a ship at sea is lying broadside on to 

 the waves, and the natural period of rolling of the ship coin- 

 cides with the period of the waves, the ship may be made to 

 roll violently, even though the waves are comparatively small. 

 For the effect of each successive wave is added to that of its 

 predecessor, and the cumulative effect is great. But if the 

 period of the waves does not coincide with the natural period 

 of the ship, the effects of successive waves neutralise one 

 another and the ship remains at rest. For example, the first 

 wave may give the ship a motion to the right, and the second 

 may strike it when it is coming back again through the equili- 

 brium position, thus destroying the motion which the first 

 wave has created. The ship thus exercises a selective action 

 on the waves, taking energy only from those the period of 

 which is approximately the same as its own. 



The resistance of a selenium cell changes when the cell is 

 exposed to the action of light. The change is found to be a 

 selective one, dependent on the wave-length of the light. If 

 the cell is stimulated in succession with monochromatic light 

 of different wave-lengths, and the change in resistance noted 

 and plotted as a function of the wave-lengths, the curve is 

 found to have a maximum for one particular wave-length and 

 to fall away on both sides of this maximum. The change can 

 be explained by supposing that in the selenium cell there are 

 electrons capable of executing vibrations about positions of 

 equilibrium. These electrons are set into vibration by the 

 incident light wave, and the amplitude of the vibrations is 

 greatest when the period of the incident light coincides with 

 the natural period of the electrons. When the amplitude 

 passes a certain critical value, the electron breaks free from 

 the point to which it is attached, and the increase in the number 

 of free electrons causes the increase in the conductivity. 



Now these three cases — tuning-fork, rolling ship, vibrating 

 electron — are all represented mathematically by the same 

 equation, namely the equation of the forced vibrations of a 

 pendulum, though in the different cases different physical 

 meanings are attached to the symbols involved. It seems 

 reasonable, therefore, to attempt to explain the selective action 

 of the eye in the same way. We shall do so here, and shall 

 begin by giving an account of a piece of apparatus that gives 

 forced vibrations and describing some experiments made by 



