Articue IV. 
Description of an Apparatus for exhibiting the Phenomena of 
the Rectilinear, Elliptic, and Circular Polarization of eaptt, 
by H. W. Baas 
From J. C. Poggendorff’s Annalen der Physik und Chemie; Berlin, 
Second Series, vol. v. p. 596. 
Upon a common tripod brass telescope-stand with a horizontal and 
vertical motion, which; from its containing a sliding-tube, may be raised 
from 16 to 25 inches by means of a tightening-screw a (Plate II. fig. 1.), 
is placed in a case / a three-sidéd-moveable brass prism be, two feet long, 
atid dividéd into Paris inches and lines. This prism carries five sliders s,, 
Sis 83, Sy, 8;, Which, by means of tightening-screws, may be fixed at plea- 
sure at any part of the scale. Two of them s,, s,, the front view of which 
is separately drawn of the actual size in fig. 2, carry stands terminating 
above in rings, which by méans of a pivot at 7 (fig. 2.) may be placed 
horizontally and vertically, so that the apertures of the Nicol’s prisms ¢¢ 
revolvable in these rings, with the centre of the convex lens 2, serewed 
into the ring of the slider s;, (the stand of the convex lens being provided 
with exactly such a pivot, and in a perpendicular position also to the 
centre of the condensing-lens p which is carried by the slider s;, the fo- 
cal distance of the condensing-léns being 12 inches and its aperture 3;) 
lie in a straight line parallel to the rod 4c, this line being at the sanie 
time the optical axis of the instramént. The Nicol’s prism of the stand sj, 
which is the nearest to this condensing-lens, may be called the pola- 
rizing, and that which is more distant from the stand s,, the avalysing 
one. 1 ! 
If parallel light is incident wpon the condensing-lens, the pola- 
rizing prism must: be in its focus, in order to polarize all the incident 
light ; if, on the contrary, the light of a lamp is employed, the pola- 
rizing prism must be in the point of convergence of the rays which fall 
divergingly upon the condensing-lens. During this process it is of course 
not the prism but the condensing-lens that is to be moved until the con- 
centrated light of the lamp falls exactly upon the aperture of the prism. 
Tn order to alter at will the planes of polarization of the two prisms, 
graduated brass plates are placed at the rings of the stands s,, s,, upon 
which plates is placed a moving index, which, when intended to be pro- 
longed backwards over the fastening-point, coincides with the longer 
diagonal of the rhomboidal bases of the Nicol’s prism. The graduation 
