transmitted through crystallized Bodies, 213 
establishes the opinion respecting the structure of the agate, 
that has been given in another part of this paper. 
2. Mica, The coloured rings are distinctly visible in mica, 
both when the light is transmitted perpendicularly through 
the plate, and when it is incident in the direction of its oblique 
depolarising axis. The irregular structure of this mineral, 
however, and the impossibility of procuring laminae with pa- 
rallel and even surfaces prevented me from investigating the 
phenomena of its coloured rings. 
3. Agate. The only plate of agate in which I have observed 
the coloured rings, is cut in such a direction that it does not 
polarise the bright image. It possesses, however, the faculty 
of depolarisation, and therefore must form two bright images 
one of whicli lies immediately above the other. This plate is 
^hout rife of an inch thick, contains no veins, and exhibits 
broad segments of coloured rings. 
4. Oriental ruby. This doubly refracting crystal affords 
beautiful rings, in which, owing to the colour of the mineral, 
the predominant colours are crimson, light blue, and bluish 
green. The central spots were distinctly visible, and though 
the crystal was of an inch thick, the rings appeared to 
be larger than those given by topaz of an inch thick. 
3. Emerald. The coloured rings formed by this stone are 
principally blue and greenish yellow, the least refrangible rays 
being extinguished by the green colour of the mineral. 
6 . Native hydrate of magnesia. This mineral affords very 
distinct segments of coloured rings when the light is trans- 
mitted in a direction nearly perpendicular to the surface of the 
laminae. Owing to the imperfect structure of the plates, I could 
not obtain a measure of the diameter of the rings. 
