Decembeb 27, 1907] 



SCIENCE 



899 



mium, 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 stand- 

 ard of length, in wave-lengths, of such 

 accuracy, that if the standard meter 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 elabora- 

 tion 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 

 dz 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 quan- 

 tities 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 quies- 

 cent ether, and the existence of this and 

 similar null results is fundamental for its 

 theory. 



He has shown the possible application 

 of the interferometer method to astronomy, 

 by himself measuring the diameters of the 

 four satellites of Jupiter, which are only 

 about one second of arc. He suggests the 

 further application of the instrument to 

 such of the fixed stars as may not subtend 



less than one hundredth of a second of arc. 



In 1898 Michelson constructed a spectro- 

 scope which enables us to make use of the 

 great resolving powers of the very high 

 orders of spectra which are absent in the 

 use of the ordinary grating, and with the 

 added advantage of having most of the 

 light in one spectrum. The echelon con- 

 sists of a pile of glass plates of precisely 

 equal thickness, which overlap by an equal 

 amount; with it spectral lines which ap- 

 pear single with the most powerful gratings 

 can be resolved into components. This 

 instrument has been especially useful for 

 the direct observation of the important, 

 because definite, influence of magnetism on 

 light, discovered by Zeeman. With thirty 

 plates, and using the 25,000th spectrum, 

 the echelon has a resolving power of 750,- 

 000, while the most powerful gratings do 

 not exceed 100,000. 



In connection with the analysis of radia- 

 tions, he has constructed and used various 

 machines for the analysis of periodic mo- 

 tions. For example, in conjunction with 

 Stratton, he perfected a remarkable ma- 

 chine which is based on the equilibrium of 

 a rigid body under the action of springs. 



Professor Michelson has also investigated 

 by his interferometer the important sub- 

 ject, both theoretically and practically, of 

 the breadth and the structure of spectral 

 lines, including the effect of a magnetic 

 field, and in various other ways his genius 

 has opened up new ground in experimental 

 optics. 



One of the Royal medals has been 

 awarded, with the approval of His Majesty, 

 to Dr. Ernest MHlliam Hobson, F.R.S. 



During the last' twenty years Dr. B. "W. 

 Hobson has been distinguished for the fun- 

 damental character of his contributions to 

 mathematics and mathematical physics. 

 His earlier published work, from 1888 on- 

 wards, deals largely with the so-called 



