332 On the Use of Nicol’s Prism. 
directions through the Nicols, and therefore to different posi- 
tions of the two planes of polarization. The reading was 
taken when the patch was halfway down the slit. This 
method of reading proved to be very sensitive. Without any 
special care I could get a number of successive readings 
whose greatest difference was 1'. This was quite sufficient 
for my purposes. Indeed my verniers were only graduated 
to 1’. But I am convinced that a far higher degree of sensi- 
tiveness could be reached if desired, by proceeding on the 
same lines. 
In securing great accuracy two precautions become very 
important. The source of light should be of uniform bright- 
ness. Here a well-made carbon filament is nearly perfect. 
Again, as much light should reach the eye from the top of 
the filament as from the bottom, abstraction being made of 
course of what is stopped by the polarizing properties of the 
Nicols. or this it is convenient that the lateral throw of 
the Nicols should be as small as possible. Flat-ended Nicols 
are therefore to be preferred. But if a pair of Nicols were 
made for the purpose, it would be easy so to slant the faces 
that the extraordinary ray should go straight through, and 
there should be no lateral throw. 
The incandescent lamp is a more powerful source of light 
for this purpose than is perhaps at first sight apparent. For 
it may be shown that with a given slit and object-glass the 
intensity of the illumination of the surface of the first Nicol 
depends solely on the intrinsic brightness of the source of 
light. The filament is practically a slit with the source of 
light brought into contact with it, so there is no difficulty 
about the whole of the surface of the Nicol being illuminated, 
and the illumination is just as powerful as would be given by 
a glowing sheet of the same brightness half an inch broad 
placed a little distance behind a slit. The intrinsic brightness 
of the filament of a Swan lamp is many times greater than 
that of a good gas-flame viewed edgeways. Besides, even if 
the gas-flame be placed very close to the slit, it is only the 
nearer portion of the flame that illuminates the whole surface 
of the Nicol. The further portion only lights up a narrow 
strip in the middle of the surface. 
I used two flat-ended Nicols, belonging to Mr. Glazebrook, 
of about half an inch aperture. One of these, which gave an 
angular throw of 15’, had a left-handed rotation-angle of 38). 
I find that the plan of reading by means of the motion of 
the patch of extinction has been the subject of an elaborate 
paper by Lippich (Sitzb. der kais. Akad. der Wissensch. 
Wien, Febr. 1882). 
