100 
Mr. J. F. W. Herschel on the action , &c. 
of the reflector is brought to conspire with that of the tour- 
maline, and the polarisation of the light which traverses it 
(which is never rigorously exact) is thereby rendered more 
complete. 
It is convenient to have sliding tubes containing lenses of 
different focal lengths according to the crystal examined, for 
the intensity of illumination of any point in the screen being, 
ceteris paribus , as the square of the focal length, consequently, 
when the rings lie within a very small angular compass, a 
greater illumination of every part of them may be obtained 
by using a lens of a longer focus. 
The dimensions of the figure, Fig. 6. PI. V. are nearly of 
the actual size. 
