678 Lord Rayleigh : Does Motion through 



metals, namely, about 2*5 XlO" 6 cm. for the upper limit. 

 The experiments on platinum films, however, would indicate 

 that for platinum films the thickness of the layer is much less. 

 According to the corpuscular theory this transition-layer would 

 be determined by A, the mean free path of the corpuscles, and 

 X has about the same value for a number of the metals*. 



The thickness at which the specific resistance of the 

 platinum film becomes abnormal cannot be taken as the 

 value of \ in the metal itself, owing to the properties of 

 the films being so very different from those of the metal. 



Summary of Results. 



(i.) The specific resistance of the films deposited in vacuo by 

 the cathode discharge is several times greater than 

 the specific resistance of the metal from which they are 

 deposited, 

 (ii.) The specific resistance of platinum films which have 

 been subjected to the same treatment remains constant 

 above a thickness of about 7 x 10~ 7 cm. Below this 

 thickness the increase of specific resistance with decrease 

 in thickness is very rapid, 

 (iii.) Heat decreases the resistance of both silver and platinum 

 films, and the thinner the film the greater the decrease. 

 In platinum films the greatest decrease is produced by 

 the electric current. 

 (iv.) The values obtained for A, the mean free path of the 

 corpuscle in the metal, are of the same order as those 

 obtained from the change of resistance produced by a 

 transverse magnetic field. 

 In conclusion my most sincere thanks are due to Professor 

 Thomson for his valuable advice and kindly interest through- 

 out the whole course of the investigation described in this 

 paper. 



Cavendish Laboratory, 

 May 12, 1902. 



L XXIII. Does Motion through the /Ether cause Double 

 Refraction ? By Lord Bayleigh, O.M., F.R.Sf 



FT1HE well-known negative result of the Michelson-Morley 

 JL experiment in which interference takes place between 

 two rays, one travelling to and fro in the direction of the 

 earth's motion, and the other to and fro in a perpendicular 

 direction, is most naturally interpreted as proving that the 



* Phil. Mag. June 1902, p. 655. 



t Communicated by the Author. Read before Section A of the 

 British Association at Belfast. 



