Page:Scientific Memoirs, Vol. 1 (1837).djvu/99

of the circle is arranged so that, in the vertical position of the stand, the straight line passing through the points 0° and 180° lies horizontally. Fig. 2. exhibits of the actual size a view of these plates, which are not drawn in fig. 1. It is preferable to graduate that side of both plates which is turned toward the eye. The dotted stand in fig. 2. is therefore to be imagined behind the plate, when it belongs to the polarizing prism, and on the contrary- before the plate and the graduation on the back of the plate, when the plate belongs to the analysing prism $d$. It will seldom be requisite to alter the plane of polarization of the incident light; it is most convenient to place it once for all horizontally, that is to say, to place the index of the polarizing prism upon 0° or 180°. In clear weather, when the light reflected by the sky is already more or less polarized, the instrument is to be directed, where this is possible, toward a wall on which the sun shines. If, however, the light reflected by the sky is to be directly employed, and in the greatest possible intensity, this may be most completely performed as follows. The polarizing prism with its plate having been placed horizontally, the analysing one is turned, until the system of rings with the black tufts is obtained in a plate of Iceland spar cut perpendicularly to the axis; within the ring $I$ of the stand $s_{4}$ the polarizing prism $e$ is then placed vertically again, and turned round until the same phænomenon is perceived in the Iceland spar. The index of the polarizing prism $e$ then indicates the direction of the plane of polarization of the incident light, and the rings appear with greater distinctness.

The light diverging from the polarizing prism is at first intercepted by a convex lens indicated by $v$, two inches in diameter, and distant 5/4 inches from the aperture $e$, and which is screwed into the lower end of that part which passes through the plate and is the holder of the prism: it then falls upon the lens three inches distant upon the stand $s_{3}$, and having 1½ inch focal distance. From this point it passes through the crystal of the stand $s_{4}$ in the ring $l$, and which is to be examined in the polarized light, and proceeds into the analysing prism $d$, into whose lower end is screwed a concave lens indicated by $u$, and of four or five inches focal distance. Any inclination to the axis of the instrument may be given at pleasure to the ring $l$, by means of a ball and socket which is represented in fig. 1., or by means of a motion on points (as in the illuminating lenses or mirrors of common microscopes). Since now the crystal in this ring may also revolve in its plane, its optical axes may be altered at will in reference to their position with respect to the plane of polarization of the incident light. If however, for the exhibition of the isochromatic curves, two crystal plates cut parallel to the axis, or two laminte of mica of uniform thickness are to be combined, the process is as follows: two turns of the screw must be given to the ring, which is to be inserted, of which the one that is represented on the larger cy-