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XXXIX. On Departures from FresneVs Laws of Reflexion. 

 By Lord Rayleigh, O.M., F.R.S* 



IN the summer of 1907, in connexion with my experiments 

 upon reflexion from glass ac the polarizing angle f, I 

 made observations also upon the diamond, a subject in which 

 Kelvin had expressed an interest. It was known from the 

 work of Jamin and others that the polarization of light 

 reflected from this substance is very far from complete at any 

 angle of incidence, and my first experiments were directed 

 to ascertain whether this irregularity could be plausibly 

 attributed to superficial films of foreign matter, such as so 

 greatly influence the corresponding phenomena in the case of 

 water J. The arrangements were of the simplest. The light 

 from a paraffin flame seen edgeways was reflected from the 

 diamond and examined with a nicol, the anode being varied 

 until the reflexion was a minimum. 



In one important respect the diamond offers advantages, 

 in comparison, for instance, with glass, where the surface is 

 the field of rapid chemical changes due presumably to 

 atmospheric influences. On the other hand, the smallness of 

 the available surfaces is an inconvenience which, however, is 

 less felt than it would be were high precision necessary in the 

 measurements. Two diamonds were employed — one, kindly 

 lent me by Sir W. Crookes, mounted at the end of a bar 

 of lead, the other belonging to a lady's ring. Xo particular 

 difference in behaviour revealed itself, 



The results of repeated observations seemed to leave it 

 improbable that any process of cleaning would do more 

 than reduce the reflexion at the polarizing angle. Potent 

 chemicals, such as hot chromic acid, may be employed, but 

 there is usually a little difficulty in the subsequent preparation. 

 After copious rinsing, at first under the tap and then with 

 distilled water from a wash-bottle, the question arises how to 

 dry the surface. Any ordinary wiping may be expected to 

 nullify the chemical treatment ; but if drops are allowed to 

 dry on, the effect is usually bad. Sometimes it is possible 

 to shake the drops away sufficiently. After a successful 

 operation of this sort wiping with an ordinarily clean cloth 

 usually increases the minimum reflexion, and of course a 

 touch with the finger, however prepared, is much worse. 

 As the result of numerous trials I got the impression that the 

 reflexion could not be reduced below a certain standard 



* Communicated by the Author. 



f Phil. Mag. vol. xvi. p. 444 (1908) ; Scientific Papers, vol. v. p. 189, 



% Phil. Mag. vol. xxxiii. p. 1 (1892) ; Scientific Papers, vol. iii. p. 496. 



