MR. G. A. SCHOTT ON THE REFLECTION AND REFRACTION OF LIGHT. 883 
or condensed gases. It is worth noting that water, which gives strong negative 
reflection when its surface is covered with grease to even a small amount, when 
perfectly clean shows hardly any elliptic polarization by reflection (Lord Rayleigh,* 
RoNTGENt). Again, various specimens of glass, whose surfaces have been repeatedly 
cleaned by a method due to Wernicke, of removing the polishing material by an 
adhesive coating of gelatine, show much greater positive reflection than when polished 
with oxide of iron or oxide of tin (Wernicke, ‘ Wied. Ann,’ 30, p. 402, and K. E. F. 
Schmidt, ‘ Wied. Ann,,’ 51, p. 417, and 52, p. 75). It is clear that the effect of a 
highly refractive surface film, either of grease or of polishing material, is to produce 
negative reflection which is superimposed ou the effect due to a gradual transition 
between the ether inside a body and that outside. This latter we should expect 
to depend on the same causes that produce dispersion and absorption (Schmidt, 
loc. cit., p. 89). Dispersion is taken account of through the refractive index. The 
absorption effect can be conveniently treated by supposing the refractive index every¬ 
where complex of the form p,(l + te). The distance in which, by absorption, the 
amplitude is reduced to 1/e of its original value is X/27rp,e. In a metal this distance 
may be as little as of ^ wave-length, in a very transparent substance such as 
glass it may be as much as 100,000 wave-lengths. These values would give 
e = about 100, about r.o uo.ooo respectively. In the one case e is large, in the other 
it is very small compared with 27rd/X, which must be less than -i-y. In considering 
such substances as glass, we may take account of quantities of order e, but may 
neglect all of higher order. 
The effect of absorption on the values of A, B, F, is of order e. 27rd/X, and may be 
neglected. The effect on D, E is of order e. 
The new value of E is 
_ 9 
A 
/^o 
2 27rd 
+ 2 (^1 — qj), 
where eQ, are the values of e for the first and second medium respectively. No 
term of order e due to the film itself occurs. Hence any small degree of opacity in 
the film changes the retardation of phase, if at all, by a whole number of wave¬ 
lengths. Wernicke ('Wied. Ann.,’ 51, p. 449) finds that whilst there is normally a 
retardation of phase of ^ wave-length, when light is reflected perpendicularly in glass 
from an opaque layer of silver closely adhering to the glass, yet the presence of a trace 
of dust or air between glass and silver sufiices to produce instead an acceleration of 
phase I wave-length. 
If the more refractive medium be also the more absorptive, as is generally the case, 
absorption increases positive reflection (since > eo) ; and of two substances having 
the same refractive index, the more absorptive will show greater positive (or less 
* ‘ Phil. Mag.’ (5), 33, p. 1. 
t ‘Wied. Ann.,’ 46, p. 152 
5 U 2 
