526 
PROFESSORS A. W. REIROLD AN^D A. W. R0CKER ON THE 
The accompanying diagram shows how far our observations can be made to accord 
with these theories. 
The point D is determined from the mean optical and electrical thicknesses of the 
black, It must, however, be remembered that we have every reason to believe that 
there are two thicknesses corresponding to this tint, and that unless the conductivity 
and the thickness are connected by a linear relation, the mean thickness of a large 
number of films will not necessarily correspond to their mean conductivity. 
The three crosses at W correspond to the three electrical thicknesses of the white 
obtained by different observations. The optical thickness is assumed to be 100 p.ju,. 
The circles indicate the extreme errors which could be due to errors in colour estima¬ 
tion. The other observations represented by A and B, and the group represented by 
C, do not require special remark. The principal deductions which can be made from 
these observations are as follows :— 
(1.) The points do not lie on a straight line inclined at 45° to the axes. Hence 
the conductivity of the interior of the film is different from that of the 
liquid in mass. 
(2.) The points A, B, D, and the centre of the group C, lie nearly on a straight 
line. If, therefore, we waive the objection above pointed out with regard to 
D, and note the fact that the group W certainly does not lie on this line, 
the facts are consistent with the theory that the film consists of an interior 
