116 Lord Rayleigh on the 



£= — — t— s C sin (mas — at) (al — 2(om)e~ ly . where mx = ?0. 



Hence, to make £=0 at each boundary, we have al = 2oom, 

 which makes ?=0, not only at the boundaries, but throughout 

 the space for which the approximate equation (22) is suffi- 

 ciently nearly true. And, putting for I 2 its value above, we 

 have 



4ft) 2 m 2 = <7 2 (m 2 -°' "p 6 *), 



whence 



m 2 = 



W 



which agrees with (16) above. 



I hope in a future communication to the Royal Society to 

 go in detail into particular cases, and to give details of the 

 solutions at present indicated, some of which present great 

 interest in relation to tidal theory, and also in relation to the 

 abstract theory of vortex motion. The characteristic differ- 

 ences between cases in which a is greater than 2w or less than 

 2co are remarkably interesting, and of great importance in 

 respect to the theory of diurnal tides in the Mediterranean, or 

 other more or less nearly closed seas in middle latitudes, and 

 of the lunar fortnightly tide of the whole ocean. It is to be 

 remarked that the preceding theory is applicable to waves or 

 vibrations in any narrow lake or portion of the sea covering 

 not more than a few degrees of the earth's surface, if for co we 

 take the component of the earth's angular velocity round a 

 vertical through the locality — that is to say, co = 7 sin I, where 

 y denotes the earth's angular velocity, and I the latitude. 



XVII. On the Resolving-power of Telescopes. By Lord 

 Rayleigh, F.R.S., Pro/ essor of Experimental Physics in the 

 University of Cambridge* . 



ALTHOUGH I have recently treated of this subject in the 

 Philosophical Magazine t , its importance induces me to 

 return to it in order to explain how easily it may be investi- 

 gated in the laboratory. There can be no reason why the ex- 

 periment about to be described should not be included in every 

 course on physical optics. 



The only work on this subject with which I am acquainted 

 is that of FoucaultJ, who investigated the resolving-powjer of 



* Communicated by the Author, 

 t Oct., Nov., and Dec. 1879, Jan. 1880. 



X " Memoires sur la construction des telescopes," Annates tie VObserva- 

 toire, t. v. ; also Verdet's Lecons d'optique physique, t. i. p. 309. 



