Page:Scientific Memoirs, Vol. 1 (1837).djvu/103

Rh between the condensing-lens and the minor, $$f$$ and $$g$$, being placed for- wards, there is seen a cooled cube upon the mirror darkened by the analysing prism, fig. 6.; and consequently when the cube is turned 45°, fig. 7, the same phsenomena are observed as if both the mica plates had been removed. Between the two mica-plates, whose axes cross each other at right angles, appears fig. 8, and indeed unchanged when the tube is turned in its ring. Fig. 9. is the complementary figure to it, which is obtained by turning the analysing prism to 90°, without changing the position of the mica plates, lf $$f$$ is bent backwards, there appears the modification of the linear figure, which produces circularly polarized incident light linearly analysed.

Of this as well as of that modification produced by circular analysis of the linear light which follows when the cube is close to the condensing-lens, it is easy to form an idea, by imagining the linear figure divided into four equal quadrants by two perpendicular lines, and the even quadrants removed from the central point about interval, and the odd ones approaching to within the same distance; or vice versâ, these removed, whilst those are made to approach. To polarize lamp-light by reflexion, the better way is to fix upon the condensing-lens (itself capable of revolving,) a mirror inclined at the fixed angle of polarization. If, by means of the polarizing prism, the instrument has before been placed upon the lamp, after the prism is turned aside and the mirror is fixed, that instrument, without changing its inclination, is turned round its perpendicular stand, until the system of rings is seen anew in the Iceland spar within the ring. Instead of employing Nicol's prisms, the light may be polarized by absorption in tourmaline plates, or by successive refractions through a series of glass plates. These are screwed into the stands in similar frames.

In order to obtain the deviation of the plane of polarization by refraction, the refracting bodies are introduced into the stand $$s_3$$. The deviation by reflexion may be conveniently observed by turning the rod at an angle toward a definite point. As, however, this (experiment is easily made in another manner, I thought it unnecessary to complicate the apparatus for it. In the same manner the apparatus may be changed into a polarizing microscope, with a still larger field of view, by the addition of some lenses and stands. But as this will be desirable in very few experiments, besides that the construction of such an apparatus by means of single rings fitting one another is easy, I omitted them in this instrument.

When a glass warming or cooling is to be examined in the polarized light, the prismatic rod is so inserted into the frame h that one of its faces which have hitherto formed the sides, is brought into a horizontal position below. All the stands are then at the side of the horizontal rod turned to 120", which presents no obstacle to the heating by