Page:Scientific Memoirs, Vol. 1 (1837).djvu/98



PON 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 $$h$$ a three-sided moveable brass prism $$b$$ $$c$$, two feet long, arid divided into Paris inches and lines. This prism carries five sliders $$s_1$$, $$s_2$$, $$s_3$$, $$s_4$$, $$s_5$$, which, by means of tightening-screws, may be fixed at pleasure at any part of the scale. Two of them $$s_2$$, $$s_5$$, the front view of which is separately drawn of the actual size in fig. 2, carry stands terminating above in rings, which by means of a pivot at $$r$$ (fig. 2.) may be placed horizontally and vertically, so that the apertures of the Nicol's prisms $$l l$$ revolvable in these rings, with the centre of the convex lens $$k$$, screwed into the ring of the slider $$s_3$$, (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 $$s_1$$, the focal distance of the condensing-lens being 12 inches and its aperture 3,) lie in a straight line parallel to the rod $$b$$ $$c$$, this line being at the same time the optical axis of the instrument. The Nicol's prism of the stand $$s_2$$, which is the nearest to this condensing-lens, may be called the polarizing, and that which is more distant from the stand $$s_5$$, the analysing one. If parallel light is incident upon the condensing-lens, the polarizing 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 polarizing prism must be in the point of convergence of the rays which fall diivergingly 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 concentrated light of the lamp falls exactly upon the aperture of the prism.

In order to alter at will the planes of polarization of the two prisms, graduated brass plates are placed at the rings of the stands $$s_2$$, $$s_5$$, upon which plates is placed a moving index, which, when intended to be prolonged backwards over the fastening-point, coincides with the longer diagonal of the rhomboidal bases of the Nicol's prism. The graduation