80 Annwersary Address by Lord Rayleigh.  [Nov. 30, 
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 are. 
In 1898, Michelson constructed a spectroscope 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 hight in one spectrum. The echelon consists of a pile 
of glass plates of precisely equal thickness, which overlap by an equal 
amount ; with it spectral lines which appear single with the most powerful 
eratings can be resolved into components. This instrument has been espe- 
cially useful for the direct observation of the important, because definite, 
influence of magnetism on light, discovered by Zeeman. With 30 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 radiations, he has constructed and used 
various machines for the analysis of periodic motions. For example, in con- 
junction with Stratton, he perfected a remarkable machine 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 subject, 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. 
RoyvaL MEDALS. 
One of the Royal Medals has been awarded, with the approval of His 
Majesty, to Dr. Ernest William Hobson, F.R.S. 
During the last 20 years Dr. E. W. Hobson has been distinguished for the 
fundamental character of his contributions to Mathematics and Mathematical 
Physics. His earlier published work, from 1888 onwards, deals largely with 
the so-called Harmonic Analysis, which embraces many topics having for 
their common aim the solution of the Potential Equation in forms suitable for 
application to the problems of Physics. The exhaustive examination of the 
general types of Harmonic Functions contained in his paper in the 
‘Phil. Trans.” 1896, has been found to be of high utility for this application. 
He was led by these researches, and particularly by the difficulty of describing 
in general terms the characteristics of a function capable of being represented 
by Fourier’s series, to take part in the revision of the logical basis of 
