PRESIDENTIAL ADDRESS — SECTION A. 41 



"" ether-wind " as it is possible to measure the velocity of the 

 wind : measuring the velocity of light in different directions. 



The result was a negative one, and this showed a fundamental 

 •difference between sonorous and luminous phenomena. 



There is, however, an important difference in the methods 

 of measuring such velocities. The velocity of light is measured 

 by exclusivelij optical devices, but that of sound is not 

 measured by an exclusively acoustic device. So long as it is not 

 possible to measure the velocity of sound by an exclusively 

 acoustic method (i.e., measuring time and lengths by ear, as, in 

 the case of light, they are measured by the eye), we have not 

 the full right to affirm the existence of that fundamental 

 difference. 



The conflict between optics and mechanics which arose from 

 the constancy of the velocity of light in vacuo, can be settled by 

 the theory of Einstein, who ascribes it to our optical methods of 

 measuring time and distance. 



As you know, the chief confirmation of Einstein's views 

 depends on astronomical observations. 



Therefore, on the road of its development, physics arrives at 

 bifurcation ; it waits that astronomy may say which of the two 

 "waj^s it shall follow. 



I have pleasure in thanking Mr. E. H. Fox for aid in 

 translating this address from Portuguese into English. 



