532 STUDIES IN ADVANCED PHYSIOLOGY. 



while the wave, however, transmitted from particle to par- 

 ticle travels with inconceivable rapidity. Speaking gen- 

 erally, light is like sound a wave motion, but these waves 

 differ in several respects from each other. In the case of 

 light the medium is ether and in sound air, or water, or 

 solids. The rapidity of transmission is very different, and 

 the form of the wave is quite unlike that of the sound wave. 

 In the transmission of a sound wave through the air the 

 particles of air carrying the wave move backwards and for- 

 wards in the direction in which the sound is moving. In 

 light, however, the individual particles of ether move at 

 right angles to the direction of the ray of light. If it were 

 possible to picture a ray of light approaching one it would, 

 using a very rough analogy indeed, look somewhat like a 

 wheel, the hub being the line of direction, while the indi- 

 vidual spokes would represent the directions in which the 

 particles of ether are thrown by the wave at that point. If 

 one could now think of such a wheel squarely approaching 

 one and imagine the spokes radiating from it in every di- 

 rection to be rapidly drawn into the hub and projected 

 again, one would have a very rough idea of the picture of 

 an approaching wave of light. 



2. The Rate of Transmission. It was impossible for a 

 long time to make actual calculations of the rate with which 

 a ray of light travels, as its rapidity was so great that for all 

 practical purposes it was instantaneous. The opportunity 

 of measuring light passing through immense distances pre- 

 sented itself when the phases of the satellites of Jupiter were 

 carefully noted. When the earth is between the sun and 

 Jupiter, that is as close to Jupiter as its orbit permits, the 

 time for the phases of one of Jupiter's satellites was carefully 

 determined. Six months later the earth is, of course, further 

 removed from Jupiter by the distance of the earth's orbit. 

 Calculations now showed that the phases of the satellite in 

 question were a little over sixteen minutes slow. The ex- 

 planation of the sixteen minutes' tardiness consisted in at- 

 tributing this loss of time to the time required by the light 



