524 Proceedings of Royal Society of Edinburgh. [sess. 
This rotation is more than is usually required in a polarimeter, 
hut it can he reduced to any desired degree; either hy turning 
the glass plate about the axis OX, so as to decrease the angle 
QOY, or by altering the vibration-direction OP of the light, 
so as to change the angle POX. 
It will be noticed that this device has the advantage over that 
of Laurent, that the same plate may be used with any colour of 
light. In illustration of the point, it may be mentioned that the 
writer is at present using the arrangement described later in a 
spectro-polarimeter, where it is giving very satisfactory results. 
The Method in Practice. 
It is obvious that there are various ways in which this half- 
shade method might be applied in practice. The author experi- 
mented with a number of different schemes, and was led to the 
conclusions now to be briefly indicated. 
In any apparatus of this kind, it is first of all requisite that 
the two halves of the field shall not be separated by a dark band, 
but shall be brought perfectly in Contact. This condition may be 
fulfilled by grinding off the top edge of the inclined glass plate so 
that it makes only a very small angle with the direction of the 
light rays. 
The diagram shows the position of the glass relative to the 
Fig. 2. — The broken line indicates the path of one of the multiple 
reflections. 
polariser. The rays of light fall on the plate at the polarising 
angle, say 57 J°. The upper surface AB is opposite to the middle 
of the Nicol, and is very slightly inclined downwards from B to A. 
This ensures that there will be no gap between the edge B and the 
upper half of the beam. On the other hand, there can be no gap 
