494 Sir Oliver Lodge on the 



Some consequences of this estimate of Density. 



14. The amplitude of a wave of light, in a place where it 

 is most intense, namely near the sun where its energy amounts 

 to 2 ergs per c.c., comes out only about 10~ 17 of the wave- 

 length. The maximum tangential stress called out by such 

 strain is of the order 10 11 atmospheres. 



The hypothetical luminous circulation-velocity, conferring 

 momentum on a wave-front, in accordance with Poynting's 

 investigation, comes out 10~ 22 cm. per sec. The arithmetic 

 of these calculations is given in the new concluding chapter 

 of ' Modern Views ' already referred to, § 6, which is likely 

 also to appear in ' Nature.' 



The supposed magnetic setherial drift, along the axis of a 

 solenoid or other magnetic field, is comparable to *003 centim. 

 per sec, or 4 inches an hour, for a field of intensity 12,000 c.G.S. 

 But it is not to be supposed that this hypothetical velocity 

 is slow everywhere. Close to an electron the speed of mag- 

 netic drift is comparable to the locomotion-velocity of the 

 electron itself, and may therefore rise to something near the 

 speed of light; say o^th of that speed: but in spite of that, at a 

 distance of only 1 millimetre away, it is reduced to practical 

 stagnation, being less than a millimicron per century. 



In any solenoid, the ampere-turns per linear inch furnish a 

 measure of the speed of the supposed magnetic circulation 

 along the axis— no matter what the material of the core 

 may be — in millimicrons per sec. 



[1 micron=10 -6 metre, 1 millimicron is 10 -9 metre = 10 -7 centimetre.] 



To get up an setherial speed of 1 centimetre per second — 

 such as might be detected experimentally by refined optical 

 appliances, through its effect in accelerating or retarding the 

 speed of light sent along the lines of magnetic force, — would 

 need a solenoid of great length, round every centimetre of 

 which 1000 amperes circulated 3000 times. That is to say, a 

 long field of four million c.G.s. units of intensity. 



15. The magnetic circulation being thus extremely difficult 

 if not hopeless to test experimentally, it may be thought that 

 by sending light down a torrent of cathode rays in a vacuum- 

 tube, a better attempt to observe some accelerating or retarding 

 effect could be made ; because here a small portion of the 

 sether is carried forward with enormous speed. But the 

 excessive tenuity of the moving portion is against the possi- 

 bility of detection. It is not the mechanical density of the 

 residual gas, as compared with that of the stagnant aether, 

 that need be considered however; — that would indeed be small, 



