98 Mr. J. F. W. Herschel on the action of 
traversing the second : the colour and intensity of this por- 
tion varying with the direction of the rav, give rise to the 
phenomena of the polarised rings, which may accordingly be 
seen by applying the eye, and receiving on it the dispersed 
light of the clouds, &c. 
In order, however, to equalize as well as disperse the 
light, which is of great importance to obtaining a perfect view 
of the phenomena, an emeried glass may be cemented on the 
anterior plate, or the first surface of the plate itself rough- 
ened ; but it will be found more convenient in practice to 
employ a double convex lens of short focus for this purpose, 
by which, if necessary, a very strong illumination may be 
obtained, and an extremely minute portion of a crystal sub- 
jected to examination. I have thus, occasionally, examined 
the rings in a portion not exceeding the hundredth of an inch 
in diameter, and thus detected irregularities of crystallization 
of a very singular nature, in many bodies, which would have 
eluded any other mode of observation. For this purpose, the 
crystal must be cemented over a small aperture in a thin 
sheet of brass, on which the focus of the lens must be exactly 
adjusted to fall.* 
If, instead of applying the eye to receive the light so dis- 
persed, we place a screen at some distance in a darkened 
room, the apparatus is converted into a solar microscope, and 
the rings will be seen projected on the screen. The con- 
* I have now an apparatus preparing, in which the first plate of tourmaline 
itself is formed into a double convex lens, by which the loss of light at two 
surfaces will be suppressed. It is easy to adapt such a lens to a double microscope, 
for the purpose of detecting microscopic irregularities ; and I have reason to sup- 
pose a variety of curious results will be brought to light by these means. 
