in the polarized Tints of certain Crystals with one axis. 341 
Table II. — Continued. 
Incidence. 
Ordinary Pencil. 
Extraordinary Pencil. 
Light yellow green. 
Good pink. 
36° 48' 
Good bluish-green. 
Good pink inclining to brick-red. 
Dull blue green. 
Orange pink. 
Very dull blue green. 
Yellow pink. 
39 41 
Pale purple, almost white. 
Pale yellow, almost white. 
4r 35' 
Pink, inclining to brick-red. 
Bluish-green, rather pale. 
Yellow pink. 
Dull pale blue. 
43 55 
White. 
Very dilute purple. 
46° 3T' 
Pale blue green,. 
Pale pipk. 
White. 
White. 
50° 2 ' 
Very pale pink. 
Very pale blue. 
White. 
White. 
Beyond this inclination, the colours are no longer distinguish- 
able. The whole series indicates a separation of the colours 
much more considerable than in the Scale of Newton ; but to 
examine the variation of the polarizing energy for the several 
simple colours more minutely, we must have recourse to homo- 
geneous light. The requisite measures were taken with every 
precaution in very fine sunshine, and though, owing to the im- 
perfections of the specimen, they do not pretend to great preci- 
sion, the resulting numbers can scarcely be erroneous to the ex- 
tent of or of their own value. I think it necessary to 
premise this, as the law of action indicated by the following 
Table of the results is so very surprising and unexpected, that 
it was not without scrupulous examination I could persuade 
myself that no enormous oversight had been committed. The 
first column expresses the colour of the incident ray, the second 
the length of the shortest period of alternate polarization it is 
capable of performing within the crystal, computed by M. Biot’s 
formula, in which t represents the thickness of 
the plate, & the angle an intromitted ray njakes with the axis 
(supposed perpendicular to the surface), n the number of pe- 
riods and parts of a period it executes during its passage through 
the plate, or the order of the ring to wfiich it is referred at its 
egress, and I the length of a period performed by the same ray, 
supposed to traverse the crystal at right angles to its axis, or 
the minimum length above mentioned. The third column con-? 
