ON THE LIMITING THICKNESS OF LIQUID FILMS. 
653 
A Fresnel’s optical bank, of tlie pattern devised by Professor Clifton, was fitted 
with the apparatus necessary to produce interference bands by means of thick plates. 
The plates were specially prepared by Messrs. Elliott Bros., and as their thickness was 
18 millims., a considerable separation of the two interfering rays could be obtained. 
The angle between the two plates of the compensator could be altered, so that the 
sensitiveness of the instrument was under control and the angular motion of the whole 
compensator could be read off' correct to 1' by means of a vernier. A small brass table 
was placed between the mirrors. It was carried by one of the sliders of the bank, and 
could be raised or lowered by rackwork. To this a brass plate, 460 millims. long by 
50 millims. broad and about 3 millims. thick, was firmly clamped. It was furnished 
with two pairs of brass V-pieces, in which glass tubes about 400 millims. long and 
18 millims. in internal diameter were placed. The ends of the tubes were ground 
and closed with plates cut from the same piece of plate glass. The requisite 
adhesion between the plates and tubes was obtained by slightly moistening the 
extremities of the latter. 
All the different parts of the apparatus were marked, so that after each readjust¬ 
ment they could be readily replaced in the positions they previously occupied. With 
this precaution it was possible to remove the tubes and set them up again many times 
in succession without displacing the interference bands from the field of view. 
The light employed was that of an oil lamp, and to prevent disturbance by heat 
from this, the apparatus was set up in front of a draught closet within which the lamp 
was placed. The window was then closed and the air surrounding the apparatus was 
thus completely cut off from that in the neighbourhood of the flame. A large screen 
of stout pasteboard prevented any light or radiant heat from the lamp or from the 
window of the draught closet falling upon any part of the apparatus except the first 
mirror and its immediate surroundings. 
Before performing an experiment the interiors of the tubes were thoroughly mois¬ 
tened with the liquid to be used, into which one of the extremities of each of them 
was then dipped. On withdrawing them plane films were formed in the tubes, which, 
if they were then inverted, ran a little way down them. A second film could then be 
formed in each by again dipping the ends in the liquid, and so on until the tubes 
contained between 50 and 60 films apiece. Being thus charged they were placed in 
the Y-pieces, closed with the glass plates, and left undisturbed until the films had 
thinned sufficiently to make an observation possible. 
During the whole of this process, each film, with the exception of the first and last 
formed, was only directly exposed to the air for the few seconds which elapsed before 
the next in order was made. A considerable quantity of liquid was retained between 
the films, so that when the tubes were closed by the glass plates the whole of the air 
within them must speedily have become saturated. In this saturated space the films 
remained for at least half an hour if formed of plain soap solution, or at least an 
hour if formed of liquide cjlycerique before they were ready for observation. The 
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