256 Mr. S. S. Richardson on 



8. Now # = 365-26-»27-32 = 13'37 andK/a = 389: so that 

 the amplitude of 3 is, in circular measure, /ex 100*9, or in 

 seconds of arc tcx2 x 10 7 . In other words, if the gravi- 

 tational indices of earth and moon differ by about one part 

 in 20 million, then the principal effect upon the moon's 

 longitude will be represented by a fluctuation with an ampli- 

 tude of one second of arc, the period being a lunar month. 

 The maxima and minima of 3 occur at times of half-moon ; 

 and if <y 1 — <y 2 is positive (that is, if the earth's gravitational 

 index is greater than the moon's), the waning moon will lag- 

 behind its theoretical (Newtonian) position. This, it may be 

 remarked, is the opposite of what would occur if a massless 

 in extensible connexion existed between earth and moon. 

 If the composition of earth and moon were definitely known, 

 there would then be the more interest in discovering whether 

 the gravitational indices of those bodies were perceptibly 

 different. But from the great difference of density we may 

 reasonably infer a wide difference of composition, and the 

 high precision which should be attainable in the comparison 

 may perhaps be held to justify a close attention to this aspect 

 of the lunar problem. 



Boar's Hill, Oxford, 

 June 1914. 



XXX. Note on Polarizing Prisms for the Ultraviolet. By 

 S. 8. Richakdson, B.Sc, A.R.C.Sc, Lecturer in Physics 

 at the Central Technical School, Liverpool *. 



IN the course of some experiments requiring a beam of 

 polarized ultraviolet rays, it was found that the prisms 

 employed transmitted no light of higher refrangibility than 

 about w.l. 3000 A.U. The polarizer and analyser were both 

 large prisms of the Foucault type, and as Iceland spar is 

 quite transparent as far as w.l. 2140 the want of: transmission 

 was due obviously to the reflexion of the extraordinary ray, 

 as well as the ordinary ray, at the air-film. This conclusion 

 was confirmed experimentally by tilting each prism parallel 

 to the principal plane of section, so that the beam should meet 

 the air-film at a steeper angle, when it was found that the 

 whole gamut of: ultraviolet rays was transmitted. Should 

 too great an inclination be given, the ordinary as well as the 

 extraordinary will be transmitted and the emergent beam 

 will no longer be plane-polarized. 



In view of this it seemed desirable to determine from such 



* Communicated "by Prof. L. R. Wilberforce. 



