Page:Scientific Memoirs, Vol. 1 (1837).djvu/100

linder enters into $l$, but the other, which is on the narrower cylinder, passes through, so that on the side toward $k$ a second crystal is screwed in, whose axis may in this manner be made to assume at pleasure any angle to the axis of the first crystal.

The ring $m$, nearly in the focal distance of <math<k</math>, is intended for the reception of cooled glasses, thin laminae of gypsum, and amethysts. Fastened to a pin, its central point is exactly in the axis of the instrument, when the pin is exactly vertical. Similar rings of wood, provided with straight pins, may be placed in the case of the stand $s_{4}$. Biaxal crystals are fastened to the pins, so that when the ring is turned round the pin, the systems of rings of the two axes pass one after another through the field of view; if therefore the indexes of the two Nicol's prisms stand at 0° and 90°, the black tufts of the systems of rings lie in a horizontal line. The ring $m$ may also serve for the reception of a micrometric arrangement for the systems of rings of the crystals observed in $l$.

In order to change the rectilinear into circular polarization, the arms $f$ and $g$, which revolve round the pegs $n$ and $o$, contain laminae of biaxal mica^[1] of such a thickness as to produce a difference of path of exactly a quarter-undulation between the two rays, when the axes of those arms $ff$ and $gg$ (Plate II. fig. 2.) form with the plane of primitive polarization $ee$ angles of 45° and 135°. Instead of the laminae of mica cooled or compressed glasses may be employed, and combined (fig. 5.) in the manner particularly described in the foregoing paper.

If the two thin plates are turned aside, the rectilinearly polarized light is rectilinearly analysed. In order to analyse circularly, the rectilinearly polarized light $f$ is brought forwards. In order to analyse rectilinearly the circularly polarized light, $f$ is to be turned aside, and $g$ placed forward. The two plates must be brought forward, as in fig. 1., when the circular polarized light is to be circularly analysed. The axis of the thin mica plate is indicated upon its frame. If that axis, instead of corresponding with the points 45° and 135°, passes through other points of graduation, we obtain the phænomena of elliptic polarization.If a small pin be fixed in the direction of the axis $gg$, the position of the axis of the lamina of mica may easily be drawn upon the graduation of the stand $s_{2}$.

In order to perform the simple experiments of intensity, it is advantageous to uncover the field of view. This is accomplished by a hollow cylinder one inch in height screwed into the somewhat projecting end of the frame of the lens $k$ up towards $m$. The aperture of the opake diaphragm in the bottom of this cylinder is 1½ line. This well-defined bright circle furnishes a very good object for these experiments. If

1. Although the same phænomena may be obtained by the determinate inclination of a thin plate of uniaxal mica, yet the employment of the biaxal mica appears tome much more convenient.