FIG. 1. 



Study of the Double Seknates of the Series E 2 M(SeO 4 ) 2 ,6H;,0. 59 



The rubidium and csesium salts have never been investigated, and 

 the author has been unable to find any evidence of their preparation. 



The section-plates and prisms employed in the optical work were 

 prepared by means of the author's cutting and grinding goniometer, in 

 the manner already described in previous memoirs. The new special 

 adjusting apparatus described to the Royal Society in the memoir in the 

 ' Phil. Trans.,' A, for 1899, p. 461, has proved of the greatest service in 

 the preparation of the prisms. All the prisms were obtained with its 

 aid by one adjustment, instead of two (one for each of the two required 

 surfaces). A very useful addition to it has been made in the shape of 

 a pair of special grip-holders, besides those referred to on page 460 of 

 the memoir just quoted. One of these new holders is represented in 

 fig. 1. The upper portion, the stem by which it is attached to the 

 lower end of the crystal-adjusting apparatus, 

 carries two vertical grooves instead of only one, 

 so that the plane within the crystal which is 

 desired to bisect the angle (60) of the prism 

 can always be preliminarily roughly arranged 

 at right angles to the 120 adjusting segment, 

 whatever be the situation of the most conve- 

 nient direction of gripping. The stem passes 

 down into a thicker solid cylindrical portion, 

 surrounded by a closely fitting hollow cylinder 

 capable of movement for somewhat over 90, 

 and fixation at any position, by means of slits 



at different levels and two clamping screws passing through them into 

 the solid cylinder. This enables the necessary final azimuth adjust- 

 ment of the crystal to be effected, so that the particular bisecting 

 plane referred to can be set exactly to the required orientation, with 

 the aid of the goniometer and its graduated adjusting movements. 

 The gripper is attached rigidly to the underside of the movable 

 cylinder. Its two prongs are arranged to be drawn together by means 

 of a screw manipulated with a milled-headed key, and the gripping 

 lower part is thickened and padded with chamois leather. The use of 

 one of these holders, which only differ in the size of the gripper so as 

 to accommodate different sized crystals, avoids the use of warm wax, 

 which may cause efflorence, or cracking, of the crystal. 



Every prism employed was prepared by the aid of this instrument 

 so that the two surfaces were symmetrical to one of the three principal 

 planes of the optical ellipsoid (indicatrix), and its edge parallel to 

 one of the two rectangular axes of the indicatrix lying in this plane. 

 It therefore yielded two of the three refractive indices directly, namely, 

 those corresponding to vibrations parallel to the two rectangular axes 

 of the optical indicatrix lying in the bisecting plane. 



The salts were prepared in the following manner : A quantity of 



