5 2 Mr. J: F. W. Herschel on the action of 
natural and artificial bodies hitherto examined, as to render 
me extremely desirous of prosecuting the research, with the 
aid of more perfect specimens, and improved methods of 
observation. 
Having arrived at the general result of a dispersion of the 
axes by the sole consideration of the gradation of tints in 
plates of various thicknesses, it becomes interesting to verify 
it by direct and independent observation. This I have ac- 
cordingly done ; and the fortunate discovery of a substance 
in which it is of enormous magnitude, puts it in our power to 
render the fact sensible to the eye of the most unpractised 
observer, by an exceedingly simple experiment, to be de- 
scribed in its place. - 
II. Of the general phenomena of crystals which develope tints 
deviating from Newton's scale, by exposure to polarised light . 
In describing the phenomena, I shall at present confine 
myself to the tints developed along the principal section of 
the crystal, which is supposed placed in an azimuth 45 0 with 
the plane of primitive polarisation. The observations of the 
tints in this position are most easily made, and least liable to 
error, and we shall see presently that it would be superflu- 
ous as well as embarrassing to examine other situations, the 
law of the phenomena being completely deducible from this. 
In this series of observations, then, we traverse the polarised 
rings (PI. V. fig. 1.) in the direction of their axis of symme- 
try AA ', passing through their poles P,P' and centre O. Now 
if we subject to this examination any one of the following 
substances : 
