1890-91.] Mr E. Sang on Nicol’s Polarising Eye-Piece. 325 
The removal of this parallax is not altogether a matter of 
necessity, it is one rather of convenience, for by turning on the 
ends of the containing box circles parallel to each other, but on 
different axes, the inconvenience of the parallax would be entirely 
removed — the adjustment, however, of these axes would be 
troublesome. 
The tendency of Iceland Spar to split in planes parallel to the 
faces of the primitive form renders almost unavoidable the employ- 
ment of rhombs whose lengths are parallel to the aretes of that 
form, economy in the amount of material being nearly as important 
as a maximum extent of field. 
Eegarding, then, the positions of the lines AB as determined by 
the cleavage, and those of AD by the condition of rectilineal trans- 
mission, there remains only to be determined the inclination of the 
plate DB of balsam. This inclination may be determined by 
attending to one or other of two conditions. Either we may so 
place this plate as to give the greatest angular field of view ; or we 
may so fix it that the verges of that field are equally inclined to 
the direction AB : practically the latter consideration is the more 
important. It will, then, be proper, before attempting the enquiry 
into the best possible form of the instrument, having regard neither 
to the economy of the material nor to the introduction of parallax, 
first to determine the form which it ought to have when influenced 
by these restrictions. 
The first thing to be determined is the angle ADC, which the 
diagonal AD of the end of the rhomb makes with the arete of the 
primitive form. Adopting the results of the elaborate investiga- 
tions of M. Malus, let (fig. 2) the whole crystal be imagined to 
occupy the point O. Suppose that KOL is the direction of the 
axis of crystallisation, and ON that of the arete of the primitive 
form, and also of that portion of the ray which is interior to the 
crystal. Describe from O, as a centre, a sphere with the radius OM 
= unit, to represent the progress of a luminous pulse in air, and 
the interior ellipsoide with its semi-axes '604 and ’673 to represent 
the luminous pulse in the interior of calcareous spar. 
In order to place the refracting surface OP in such a position 
that the pencil of extraordinary light may not be bent, we must 
apply tangent planes at the points M and N, and continue these 
VOL. xviii. 8/12/91. 2 n 
