1887.] 
Professor Tait on Glories. 
359 
Hence, the only possible explanation is diffraction depending 
on the form of the vertex of the reflected wave. The form of 
an originally plane wave, once reflected inside a drop of water is, 
roughly, when the central ray has just emerged, a portion of an 
hyperboloid of revolution, doubled back cusp-wise round its border. 
An approximate calculation is given, based on this assumption. 
A simple first approximation to the theory of glories is given 
by the behaviour of a plane wave incident normally on a screen 
pierced with a great number of very small circular apertures of 
nearly equal size. They are thus, to a certain extent, analogous to 
coronse. 
8. Report on the Pennatulida, dredged by H.M.S. “ Porcu- 
pine.” By A. Milnes Marshall, M.D., D.Sc., M.A., 
P.R.S., Beyer Professor of Zoology in the Owens 
College, and by G. H. Fowler, B.A., Ph.D., Berkeley 
Fellow of the Owens College, Manchester. Communi- 
cated by John Murray, Esq., Ph.D. 
Friday , 15 th July 1887. 
JOHN MURRAY, Ph.D., Vice-President, in the Chair. 
The following Communications were read : — 
1. Stability of Fluid Motion. — Rectilineal Motion of Viscous 
Fluid between two Parallel Planes. By Sir W. Thomson, 
LL.D., F.R.S. 
27. Since the communication of the first of this series of articles 
to the Royal Society of Edinburgh in April, and its publication in 
the Philosophical Magazine in May and June, the stability or in- 
stability of the steady motion of a viscous fluid has been proposed 
as subject for the Adams Prize of the University of Cambridge for 
1888.* The present communication (§§ 27-40) solves the simpler 
* See Phil. Mag., July 1887. 
