556 Prof. A. A. Michelson on Metallic 



green, or even bluish green when the illumination is sufficiently 

 oblique*. 



The chief characteristics by which "metallic" reflexion 

 may be distinguished may be summarized as follows: — 



1. The brightness of the reflected light is always a large 

 fraction of the incident light, varying from 50 per cent, to 

 nearly 100 per cent. 



2. The absorption is so intense that metal films are quite 

 opaque even when their thickness is less than a thousandth 

 of a millimetre. 



3. If the absorption varies with colour, that colour which 

 is most copiously transmitted will be the part of the incident 

 white light which is least reflected — so that the transmitted 

 light is complementary to the reflected. 



4. The change of colour of the reflected light with changing 

 incidence has already been mentioned. It follows the in- 

 variable rule that the colour always approaches the violet 

 end of the spectrum as the incidence increases. If the colour 

 of the normal reflexion is violet the light vanishes (changing 

 to ultra-violet), and if the normal radiation be infra-red it 

 passes through red, orange, and yellow as the incidence 

 increases. 



While the criteria just considered are the simplest and 

 most convenient for general observation, it is to the more 

 rigorous results of more refined optical methods that we must 

 look for the final test of the quality of reflexion in any given 

 case; to determine whether or not a. colour phenomenon 

 mav be due to " metallic " reflexion or to one of the other 

 general causes. 



Such optical tests are furnished by the effect of reflexion 

 upon polarized light. The elements ol the resulting elliptic 

 vibrations may be expressed in terms of the amplitude ratio 

 R oE the components, and of the phase difference P corre- 

 sponding to the angle of incidence I, as in the following 

 tables for silver and for glass. 



The very marked difference in the run of the numbers in 

 these tables may be rendered still more striking by plotting the 

 values as ordinates of: the curves shown in PL IV. fig. 1, which 

 gives at a glance the form ofthf* "phase" curve (full line) and 

 the " amplitude " curve (dotted line) for silver, steel, graphite, 

 selenium, flint glass, crown glass, and quartz. It is evident 



* The change in colour is very much more marked when the light is 

 polarized perpendicularly to the plane of incidence. As the angle of 

 incidence approaches the angle corresponding to the "polarizing angle" 

 the colour is a deep blue or even purple. 



