) #907. | Anniversary Address by Lord Raylezgh. 79 
MEDALLISTS, 1907. 
CopLEY MEDAL. 
The Copley Medal is awarded to Professor Albert Abraham Michelson, 
For. Mem. R.S., on the ground of his experimental investigations in Optics. 
In 1879, Michelson brought out a determination of the velocity of light 
by an improved method, based on Foucault’s, which gave 299,980 kilometres 
per second. Three years later, by means of a modification of the method, 
capable of even greater precision, he found for this constant, of fundamental 
importance for-electric as well as optical science, the value of 299,853 
kilometres. 
Michelson has been a pioneer in the construction of interferometers, which 
are now indispensable in Optics and Metrology. With his new instrument, 
at Paris, he determined the absolute wave-lengths of the red, green, and 
blue lines of cadmium by counting the number of fringes (twice the 
number of wave-lengths) corresponding to the length of the standard metre 
of the Bureau International des Poids et Mesures. He found the metre to 
be 1,553,164 times the wave-length of the red line of cadmium, a result 
which is almost in exact agreement with the redetermination last year by 
Perot and Fabry. Michelson thus proved the feasibility of an absolute 
standard of length, in wave-lengths, of such accuracy, that if the standard 
metre were lost or destroyed, it could be replaced by duplicates which could 
not be distinguished from the original. 
He had the greatest share in the elaboration of precise experiments on the 
relative motion of ether and matter. He repeated in an improved form 
Fresnel’s experiment of the speed of light in moving media, using water and 
sulphide of carbon. He found that the fraction of the velocity of the water 
by which the velocity of light is’ increased is 0-434, with a possible error of 
+ 0°02. The fact that the speed is less in water than in air shows 
experimentally that the corpuscular theory is erroneous; but his results, 
moreover, established the correctness of Fresnel’s formula for the effect, the 
theory of which has since become well understood. 
In conjunction with E. W. Morley, he devised and carried out a very 
remarkable method by which, on the assumption of ether at rest, an effect 
depending on quantities of the order (v/V)? would appear to be appreciable. 
No displacement of the fringes was found. Of this result the simplest 
explanation would be that the ether near the earth partakes fully in its 
orbital motion ; but modern electric and optical science appears to demand a 
quiescent ether, and the existence of this and similar null results is funda- 
mental for its theory. | 
