349 
two parts are cemented together, as in Fig. 2, with the surfaces of sec- 
tion in contact, and the ends of the prism thus formed are 
then ground and polished. 
Now, it is evident from the construction of the prism, 
that if two rays of light parallel to the axis be made to 
traverse the two parts of the prism respectively, the lines 
of separation of the ordinary and extraordinary images 
in these two parts will be CA, CA’, Fig. 3, and as the 
angle between the plane of section and the longer diago- 
nal is small, the angle ACA’ is nearly 180°. Hence the 
extraordinary refractions in the two parts are in nearly Fig. 2. 
opposite directions, and if the end at which the 
light is admitted be so chosen that these refrac- 
tions shall be from the plane of section, the sepa- 
ration of the images will be nearly doubled. 
Now, suppose a circular beam of plane- 
polarized light to traverse the prism in a direc- 
tion parallel to the sides, and so as to be equally a’, 
divided by the plane of section, the emergent ; 
beam will consist of three separate parts, viz.: 
1, a circular beam, OCO’, Fig. 3, formed by the 
union of the two ordinary beams; 2, two semi- 
circular extraordinary beams, E, EK’. Ifthen the 
size of the incident beam be suitably determined, 
these latter may be completely separated from 
the ordinary beam, so as to admit of their being 
stopped by a diaphragm which allows the ordi- 
nary beam to pass; the instrument will then 
transmit a single beam of plane polarized light. HG, & 
Now, it is easy to see that the planes of polarization of the two parts 
into which the beam is divided by the plane of section are inclined to 
each other at an angle somewhat less than double the angle DCS, Fig. 
1. Suppose, then, that the plane of the paper being perpendicular to the 
beam, the traces of these planes of polarization are represented by AB, 
A'B’, and let CP, CP’ be perpendicular to these lines respectively. Let. 
Cp be the plane of polarization of the ray : 
to be examined. Then, so long as there is AMA 
any difference between the angles PCy, 
P'Cp, the intensities of the two parts of the 
beam will be different, and conversely, if 
these intensities be equal, it is evident that 
the required plane of polarization will bi- 4, By 
sect the angle PCP’. The prism must, nM upeoneeser ot z 
therefore, be turned on its axis until the Fig. 4. 
equality of tints be established, and when 
this is done, the position Cy of the plane of polarization is known. It is 
not, however, necessary to determine the position of the planes PC, P’C. 
The observer commences by transmitting a beam whose plane of polari- 
