366 A. W. Ewell — Rotatory Polarization. 



p, where it enters the observer's eye. The upper Nicol, p, is 

 mounted in the tube, q, which rotates freely within the tube, 

 w, attached to the gear wheel, o, and a graduated circle, t. 

 The position of this Nicol is read with a pointer, v. The lower 

 Nicol, c, and the biquartz, d, are fixed in a tube which is 

 attached to the gear wheel, b. The upper gear wheel, o, and 

 graduated circle, t, can be rotated in step with the lower gear 

 wheel, &, the lower Nicol, c, and biquartz, r/, by means of the 

 rod, u, with its small attached gears. Thus the azimuth of 

 polarization of the entering light, with reference to any par- 

 ticular radius of the jelly tube, may be varied without chang- 

 ing the relative position of the lower Nicol and the graduated 

 circle of the analyzer. The circle, t, has a second graduation 

 along the outer edge and the azimuth is read by means of the 

 index, s. z represents a jelly tube of the form described 

 above, m is the brass tube, e the glass disk and Jc the binding 

 wire. The upper brass tube is clamped in the fixed clamp, n 

 (the position of this clamp can be varied to suit different 

 lengths of tube). The lower brass tnbe is fastened in the 

 movable clamp, j. A graduated circle, v, is attached to the 

 bottom of this clamp and the amount of twist applied to the 

 tube is read with the index, h. The twist was generally pro- 

 duced by pulling the cords, a?, by weights, or by hand, and 

 then clamping the cords at A. The pulleys, y, had cone-bear- 

 ings and turned with no appreciable friction. Their position 

 could readily be varied. The lower clamp could be clamped 

 in position by the clips, g, to the platform, <?, which could be 

 raised or lowered by the screws, f. i represents the cross 

 section of a ring soldered to the platform, e, which fitted loosely 

 within the lower clamp and served to keep it from being drawn 

 too far to one side during adjustments. 



Signs. — The right-handed screw notation is adopted.* If, 

 advancing in any direction, the direction of rotation of the 

 plane of polarization or the direction of twist of the body be 

 related to the direction of advance as the direction of rotation 

 of a right-handed screw is related to its advance, the rotation 

 or twist is said to be positive while the opposite twist or rota- 

 tion is called negative. The graduated circles being numbered 

 in the usual direction, an increase in the reading means a nega- 

 tive rotation or twist. Twists and rotations are throughout 

 expressed in degrees. Degrees are given to the nearest whole 

 number. The accuracy does not warrant stating fractions, 

 even in averages. 



Double Refraction. — A double refracting body may rotate 

 the plane of polarization of linear polarized light. Double 

 * Maxwell, Elec. and Mag., 3d ed., i, p. 25. 



