APPARATUS FOR EXHIBITING THE POLARIZATION OF LIGHT. 87 



of tlie circle is arranged so that, in tlie vertical position of the stand, the 

 straight line passing through the points 0° and 180° lies horizontally. 

 Fig. 2. exhibits of the actual size a view of these plates, which are not 

 drawn in fig. 1. It is preferable to graduate that side of both plates 

 which is turned toward the eye. The dotted stand in fig. 2. is there- 

 fore to be imagined behind the plate, when it belongs to the polarizing 

 prism, and on the contrary- before the plate and the graduation on the 

 back of the plate, when the plate belongs to the analysing prism d. It 

 will seldom be requisite to alter the plane of polarization of the incident 

 light; it is most convenient to place it once for all horizontally, that is 

 to say, to place the index of the polarizing prism upon 0° or 180°. In 

 clear weather, when the light reflected by the sky is already more or less 

 polarized, the instrument is to be directed, where this is possible, toward 

 a wall on which the sun shines. If, however, the light reflected by the 

 sky is to be directly employed, and in the greatest possible intensity, 

 this may be most completely performed as follows. The polarizing 

 l^rism with its plate having been placed horizontally, the analysing one 

 is turned, until the system of rings with the black tufts is obtained in a 

 plate of Iceland spar cut perpendicularly to the axis ; within the ring I 

 of the stand S4 the polarizing prism e is then placed vertically again, and 

 turned round until the same phaeuomenon is perceived in the Iceland 

 spar. The index of the polarizing prism e then indicates the direction 

 of the plane of polarization of the incident light, and the rings appear 

 with greater distinctness. 



The light diverging from the polarizing prism is at fii-st intercepted 

 by a convex lens indicated by v, two inches in diameter, and distant 4 

 inches from the aperture e, and which is screwed into the lower end 

 of that part which passes through the plate and is the holder of the 

 prism : it then falls upon the lens three inches distant upon the stand s.^, 

 and having H inch focal distance. From this point it passes through 

 the crystal of the stand s^ in the ring I, and which is to be examined 

 in the polarized light, and proceeds into the analysing prism d, into 

 whose lower end is screwed a concave lens indicated by w, and of four or 

 five inches focal distance. Any inclination to the axis of the instrument 

 may be given at pleasure to the ring /, by means of a ball and socket 

 which is represented in fig. 1., or by means of a motion on points (as in 

 the illuminating lenses or mirrors of common microscopes). Since now 

 the crystal in this ring may also revolve in its plane, its optical axes may 

 be altered at will in reference to their position with respect to the plane 

 of polarization of the incident light. If however, for the exhibition of the 

 isochromatic curves, two crystal plates cut parallel to the axis, or two 

 laminte of mica of uniform thickness are to be combined, the process is 

 as follows : two turns of the screw must be given to the ring, which is 

 to be inserted, of which the one that is represented on the larger cy- 



