THE ELECTRICAL RESISTANCE OP THIN LIQUID FILMS. 
4G1 
effected without endangering its existence, and tlieir position so adjusted that the 
points at which they meet the film may lie upon one of the vertical bands of light 
seen from the cathetometer. 
Method of measuring the angle of incidence .-—In Plates 59, 60, figs. 1 and 5, NN is 
a stout mahogany board which supports the whole apparatus. It has the form of an 
incomplete circle, the centre of which coincides with the centre of the cup B, and a 
paper scale graduated to half degrees is attached to its outer edge. It rests upon a 
long wooden slab SS, fixed in the window recess, by three points, viz. : the knob q, 
and the two levelling screws p, p. A portion of the slab SS has been cut away to 
make room for the lower chamber. 
T is the telescope of the cathetometer, placed at a distance of about 2 metres from 
the film, and Q and It are the mirrors for reflecting light upon the film. The angle of 
incidence in the case of either mirror was determined in the following manner :— 
In the accompanying figure, let B D represent the film and AFC the graduated 
circle, and let the light fall on the film in the direction A B and be reflected along 
B C. Join 0 A, O C. 
If 
then 
whence 
OA(=30'4 centims.) = a, the angle AOB=a 
0B(=1'65 centim.) =h, ,, ,, ABF = 0 
a sin 6 
b sin(0 — of 
. n a sin a 
tan v— - - 
a cos « — b 
and thus 6 is known if a is known. 
3 0 2 
