Prisms for Polarizing -Purposes. Ill 
to cut me a prism with its axis at right angles to the optic 
axis, when they informed me that they believed their late 
father, so well known for polariscope apparatus, had cut some 
such prism to order many years before. They had an impres- 
sion it was for Professor Stokes; but they had no clear 
recollection about it. If it did fail, as they implied, it was 
probably for want of a cementing medium which could take 
advantage of the full interval between the indices, as explained 
by Hartnack in his admirable essay. I trust that Professor 
Thompson, in his forthcoming paper, will collect for English 
readers what has been done abroad, and describe in an acces- 
sible form the prisms of Senarmont, Rochon, Grlan, and others. 
Meantime I wish chiefly to point out that, if the full ad- 
vantage of these improved prisms is to be had, undue length 
must be avoided. Most JSTicol prisms are longer than they 
need be ; and Hartnack by no means does justice to his own 
prism in this respect, giving its length as over three times 
the width. The Rev. P. P. Sleeman possesses a prism only 
2*75 times its width, cut as below, which performs admirably 
in projection, transmitting a much larger pencil and covering 
more field. Again, a polarizer can and ought to be made 
shorter than an analyzer, since it only has to transmit rays ap- 
proximately parallel ; hence the reason we can use a Foucault 
as polarizer but not as analyzer, owing to its small angle of 8°. 
But a more important point still in getting field, especially 
for microscope polarizers, is this : — A Nicol section is made 
from the corner B of the spar to the 
opposite corner, the end elevation 
being the rhombus ABCD. For 
this there are practical reasons, the 
cut being made through a natural 
piece of the spar. But in the 
Hartnack, Glazebrook, or Thompson 
prisms, where all the sides and cuts 
are artificial, this reason does not 
exist. Hence Prazmowski, in cutting his prisms from corner 
to corner like a Nicol, threw away a great deal of field. 
This is not theory only; it has been seen by Steeg and 
Reuter, who make their Hartnack prism of the very same 
length and angle, with the end elevation E F Gr H, the section 
being from the edge E F to the opposite one. A glance will 
show that a pencil of double the sectional area will thus 
be passed through a prism of the same section-angle and 
length. Apologizing for drawing attention to these practical 
considerations, I am &c., 
Wellfield, Ashley Road, LEWIS WEIGHT. 
Crouch Hill, N. 
K2 
