88 DOVE’'S DESCRIPTION OF AN APPARATUS 
linder enters into J, but the other, which is on the narrower cylinder, 
passes through, so that on the side toward & 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 , is intended for the re- 
ception of cooled glasses, thin laminz of gypsum, and amethysts. Fast- 
ened 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,._ 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 O° 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 ar- 
rangement for the systems of rings of the crystals observed in /. 
In order to change the rectilinear into circular polarization, the arms 
fand g, which revolve round the pegs 7 and 0, contain lamine of biaxal 
mica* 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 laminz of mica cooled or com- 
pressed 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 fis brought forwards. In-order to analyse rectilinearly 
the circularly polarized light, f is to be turned aside, and g placed for- 
ward. 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 phenomena 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,. 
In order to perform the simple experiments of intensity, it is advan- 
tageous 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 14 line. This well-defined 
bright circle furnishes a very good object for these experiments. If 
* Although the same phenomena may be obtained by the determinate incli- 
nation of a thin plate of uniaxal mica, yet the employment of the. biaxal mica 
appears tome much more convenient. 
