54 Mr. J. F. W. Herschel on the action of 
loped in the poles, or along the apparent axes of the crystal, 
descends in the scale of colour, as the thickness of the plate 
increases, and vice versa. In very small thicknesses, the tints 
approximate pretty closely to Newton's scale, or wholly 
coincide with it, while in very great ones, the tint developed 
in the poles is the composite white of the extremity of the 
scale. The angular distance, however, of the virtual poles 
from each other and from the axes, remains absolutely un- 
changed for all thicknesses ; and this striking fact, which I have 
proved by numerous and satisfactory experiments, was first 
suggested for examination as a result of theory, and would 
equally hold good, as will presently be proved, for every con- 
ceivable law of double refraction. 
The substances which I have examined most attentively, are 
sulphate of baryta, nitre, mica, and Rochelle salt, and the sub- 
joined tables of tints developed for different inclinations in 
plates of the first and last of these, may serve as examples 
of the mode of action of the respective classes to which they 
belong on light, and will afford data for some calculations 
to follow. The first two columns contain the inclinations 
corresponding to similar tints of the incident ray on the 
moveable plate which carries the crystal, in the general ap- 
paratus imagined by M. Biot, for observations of this kind. 
Were the plate cut in a direction precisely perpendicular to 
the optic axis, (or line bisecting the angle between those of 
double refraction) and adjusted with perfect accuracy on the 
instrument, the excesses or defects of these angles above or 
below go° would represent the angles of incidence. Neither 
of these conditions were, of course, exactly fulfilled. But it 
is obvious that the small errors in these particulars (which 
