114 Trans. Acad. Sci. of St. Louis. 



to a distant fixed one, and is reflected back to the revolving 

 one again, which has appreciably moved during this to and 

 fro passage of the light. The beam emerging from the 

 revolving mirror will be displaced from the entering beam, 

 by an amount which will increase with the angular velocity 

 of the mirror, and the distance between the two mirrors. 

 The intermittent light thus reflected was also focused upon 

 the tooth of a cogged wheel. When the wheel was driven at 

 such a speed that successive teeth appeared at the focus at 

 intervals equal to that of a rotation of the mirror, the wheel 

 would seem to be at rest. If the wheel were slowed down 

 slightly, it would seem to be rotating slowly in a direction 

 opposite to that in which it was moving. If the wheel were 

 slightly accelerated, it would have a slow apparent motion in 

 the direction of its actual motion. This method gives a very 

 accurate measurement of the time of rotation of the mirror 

 and involves determining the number of rotations per second 

 of the cogged wheel. 



Michelson has made great improvements in Foucault's 

 methods. With a slower rotation of the mirror, he obtained 

 very much greater deviations of the returning beam. His 

 first announcement of preliminary results was made in this 

 city in 1878. In his final work the velocity of light was de- 

 termined with a possible error of two hundredths of one per 

 cent. This result has been universally accepted as the best 

 attainable value. 



The medium which transmits light is also concerned in the 

 transmission of electrical and magnetic action. Faraday 

 paved the way for this idea. He did not indeed concern him- 

 self with the nature of the ether, but he did abandon wholly 

 the idea of action at a distance, which had formed a sufficient 

 basis for mathematicians like Poisson and Gauss. His work 

 between 1831 and 1841 resulted in establishing the idea that 

 inductive action is communicated from point to point in space. 



In 1850 Lamont of Munich established a periodicity in the 

 average amount of daily oscillation in the magnetic needle. 

 This fluctuation was due to a periodic change in the frequency 

 of what have been called magnetic storms, with their at- 

 tendant auroral displays. In 1851 Schwabe of Dessau es- 



