466 Prof. Dove on some Phcenomena 



refracting prism, when the rotation is slow, it constantly ex- 

 hibits figures of the same brightness. The intense comple- 

 mentary colours of their overlapping margins, on introducing 

 a lamina of selenite, mica, or a thin layer of calcareous spar, 

 completely disappear. The complementary system of rings 

 of calcareous spar and apophyllite, ground at right angles to 

 the axis, the splendid isochromatic curves of nitre, ground at 

 right angles to the line bisecting the angle formed by the two 

 axes, completely neutralize each other, becoming white. The 

 same applies to the colours of unannealed and compressed 

 glass. The centre of the system of rings which in rock- 

 crystal (on slowly rotating the polarizing apparatus) passes 

 through the colours of the spectrum, appears perfectly colour- 

 less on rapid rotation. 



We thus have in the rotating polarizing apparatus, a circle 

 of colours for the complementary colours ; and not only for 

 those which occur in Newton's rings, but also for the compli- 

 cated system of rings of those crystals the binary axes of 

 which are either situated in one plane with several lines of 

 bisection, or in different planes with the same line of bisection 

 (selenite, borax). 



We may easily convince ourselves of the true existence of 

 the polarization on the one hand, and the coloured curves on 

 the other, however rapid the rotation may be. We need only 

 place a Leyden jar, which spontaneously discharges itself, in 

 the situation of the lamp, to illuminate for an instant the Ni- 

 chol's prism rotating in the dark, and to convert the rotation 

 into apparent repose*. With monochromatic light, the dark 

 lines of interference vanish in a uniform illumination. If the 

 red system of rings be separated from the violet in a binaxial 

 crystal by a glass coloured by cobalt, both disappear during 

 the illumination in an indefinite coloured mixture, in which 

 it is indifferent whether, as in nitre, the red axes form the 

 lesser angle, or, as in carbonate of lead, the violet. 



* If light which has been perfectly polarized by reflexion from a mirror 

 be transmitted first through an unannealed glass and then through two glass 

 appendages, of which the refracting plane of one is situated in the reflecting 

 plane of the polarizing mirror, the refracting plane of the other being at 

 right angles to it, the colours of the cooled glass entirely vanish and it ap- 

 pears white, provided each of the two analysing apparatus is so inclined to 

 the incident ray as to polarize an unpolarized incident ray to the same 

 extent. With the slightest alteration of either apparatus tiie complement- 

 ary images inmiediately appear, when the figure previously appeared co- 

 lourless. This at once affords a very simple means of comparing tlie power 

 of two polarizing apparatus, based upon simple refraction, and of illustrating 

 the views of Sir David Brewster on the dependence of the polarization 

 upon a number of plates, their refractive power, and the incidence of the 

 light. 



