Bauhaus-Universität Weimar

One of these rectangular pieces is then inserted in the refractometer in the usual manner, 
a little monobromonaphthalene being first placed on each surface. An attempt is then made 
to find a boundary setting but, owing to the presence of two such regions which overlap, the 
dark and light portions of the field are separated by a patch of intermediate brightness. The 
use of a nicol prism above the eyepiece enables each of the boundaries to be examined 
separately, so that two readings for refractive indices may be taken. 
The second rectangular piece is then tested in the same way, one of the values obtained 
being identical with one of those previously found. 
In the accompanying diagram, AB is the trace of one extinction plane, and CD that of the 
other. The vibration directions shown at B are those corresponding to the two indices 
found from the strip whose length is parallel to AB. The directions shown at C arc those 
for the determinations made with the strip whose length is parallel to CD. These directions 
can be checked by noticing the setting of the nicol while the refractive indices are being 
The following figures were obtained in the course of a determination : 
For the strip whose long edges were parallel to AB, 1-5632 and 1-5971 ; for the second strip, 
1-5632 and 1-6021. 
The refractive indices are : 
For vibrations normal to the plate, 1-5632; 
For vibrations parallel to CD, 1-5971 ; 
For vibrations parallel to AB, 1-6021. 
AB was the trace of the plane containing the optic axes (Kohlrausch’s values are 1-5609; 
1-5941 ; and 1-5997). 
If a rectangular plate be cut at random, a value is obtained for vibrations along the normal 
to the plate, and also one which is intermediate between the two other principal indices. 
When a plate of a uniaxial crystal is tested in this manner, the vibration directions can 
only be found from observations on the settings of the analysing nicol. 
PANEL. By A. R. SMELLIE, The Institute of Physiology, University of Glasgow. 
[A/5, received 4lit December, 1931.] 
The Brodie clock as at present constructed suffers from the obvious disadvantage that it 
allows the record of only one set of intervals at a time. The following modification overcomes 
this disadvantage. 
The adjustable spring contact of the Brodie clock is removed entirely and replaced by a 
broad brass plate. A piece of ebonite is fixed to the lower edge of this plate and carries 
five light platinum-tipped steel springs. Each spring is placed opposite one of the five 
striking points of the clock wheels. Five platinum-tipped screws are let into the upper 
edge of the brass plate opposite the springs. 
When the striking point of a wheel comes round, it touches a vulcanized fibre pawl 
attached to the spring and pushes the upper platinum extremity of the spring against the 
corresponding screw. 
The distributing panel is attached to the wall close to the clock and carries five brass 
strips, each connected to the lower end of one of the steel contact springs. Each strip is 
drilled with four taper holes. The outgoing wires to the time markers end in flexible wires 
provided with taper pegs to fit the taper holes in the brass strips. Insertion of a peg into


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