378 Mr Pocklington, On a method of increasing the 
where c, f are the ratios in which the amplitude is changed by 
the refractions into and out of glass. Since 6 2 + c/= 1, these can 
be written 
A cos (f > . 
2M1-VJ 1 
(1 + 6: 2 ) 2 ’ 
A 
sin <f> . 
2b, (1 - h 2 J 
(1 + b 2 J 
where b x — sin (i — ?’)/sin (i -f r), 
and b 2 = tan (i — r )/ tan (i + r). 
If i = 45° and g=Vb these parts of either beam are 
•38 A cos (f) and *18 A sin (f > , and to make these equal we must 
have cj) = 65°. 
4. The main objection to be urged against this method of 
obtaining two circularly polarised beams is that it requires that 
the interval between the two inclined plates shall be very 
accurately adjusted. Whether it is better to use this method, or 
use the ordinary form of interferometer (with unsilvered inclined 
plate) and allow for the want of exactitude of circularity in the 
polarisation when calculating the result, can only be determined 
by experiment. 
5. The phenomena in either case are rendered less brilliant 
by the light reflected from the other face of the plate. This light 
can be eliminated either by making the plates slightly wedge- 
shaped, or by using a circular hole in a screen as a source of light 
and forming a real image of it beyond the apparatus by means of 
a lens. The light to be eliminated now passes through an image 
distinct from that formed by the light which we want, and can be 
cut off by a screen. 
[Note, Received 14 March 1902.] 
One of the most sensitive Polaristrobometers is Laurent’s 
form. In this the light in half the field has its plane of polarisa- 
tion rotated through a certain small angle. On rotating the 
analyser first one half and then, after a further small rotation, the 
other half of the field becomes dark. The source of the great 
sensitiveness is partly that the ratio of the two intensities varies 
from 0 to oo for a small rotation of the analyser, and partly that 
this rotation can be accurately measured. 
The same principle can be used with the interferometer with- 
out utilising circularly polarised light. The mirror should be 
