86 



Article IV. 



Description of an Apparatus for exhibiting thePhcenomena of 

 the Eectilinear, Elliptic, and Circular Polarization of Light, 

 A^H. W. Dove. 



From J. C. Poggendorff's Annalen der Physik und Chemie ; Berlin, 

 Second Series, vol. v. p. 596. 



U PON a common tripod brass telescope-stand with a horizontal and 

 vertical motion, which, from its containing a sliding-tube, may be I'aised 

 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,„ *5, s„ Sj, 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 means of a pivot at r (fig. 2.) may be placed 

 horizontally and vertically, so that the apertures of the Nicol's prisms 1 1 

 revolvable in these rings, with the centre of the convex lens A, screwed 

 into the ring of the slider 5^, (the stand of the convex lens being provided 

 with exactly such a pivot, and in a perpendicular position also to the 

 «entre of the condensing-lens j9 Avhich is carried by the slider «„ the fo- 

 cal distance of the condensing-lens being 12 inches and its aperture 3,) 

 lie in a straight line parallel to the rod h c, this line being at the sanie 

 time the optical axis of the instrument. The Nicol's prism of the stand ^5^, 

 which is the nearest to this condensing-lens, may be called the pola- 

 rizing, and thiat which is more distant from the Stand s^, the aVmlysvri^ 

 one. 



If parallel light is incident upon the condensing-lehs, the poki- 

 rizing prism must be in its focus, in order to polarize all the incidertt 

 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. 



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^, 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 



