8 
DE. A. E. H. TUTTON ON THE 
Extinction determinations with two sections parallel to the symmetry plane afforded 
the following results :— 
Inclination of 2M.L. from normal to c {001}. 
Plate 1.11° 28' Plate 2.12° 2' 
Mean . . . ll c 45' 
The direction is behind the normal, towards the vertical axis c. 
Murmann and Rotter found for this angle 10° 58', 
and Senarmont 12° 1 '. 
As the axial angle ac is 73° S', this 2M.L. 
extinction direction is 5° 12' in front of the vertical 
axis c, and the first median line is 11° 45' from 
the inclined axis a, both extinction directions in 
the symmetry plane thus lying in the obtuse axial 
angle ac. 
The next table shows the positions of the a 
extinction direction in all four nickel salts, and fig. 3 
illustrates them graphically. 
Inclinations of a extinctions of Ni salts in front of axis c. 
(2M.L. for Am, K, and Rb salts, 1M.L. for Cs salt.) 
AmNi Sulphate 5° 12 ; RbNi Sulphate 12° 38' 
KNi „ 8° 42' CsNi „ 24° 7' 
The position of the ammonium salt ellipsoid is such that its a axis lies nearest to 
the c axis, and the ellipsoid rotates as the potassium, rubidium, and caesium salts are 
successively reached, further and further from this position, the rotation following the 
order of the atomic weights of the three alkali metals. 
Refractive Indices. —Six 60°-prisms were ground by means of the author’s cutting 
and grinding goniometer,* each to afford two indices directly, and the results are 
* A new crystal-grinding goniometer on the Goldschmidt two-circle principle has recently been 
described by F. E. Weight (‘Journ. Washington Acad, of Sciences,’ 1915, 5, 35). In its description a 
mere passing mention is made of the very efficient instrument which the author (Tutton) described in 
the year 1899 (‘Phil. Trans. Eoy. Soc.,’ A, 1899, 192, 457 ; ‘ Zeitschr. fur Kryst.,’ 1899, 31, 458; 
‘ Crystallography and Practical Crystal Measurement,’ 1911, p. 682), and which has been used in all his 
investigations during the past 16 years, over a thousand section-plates and prisms having been most 
accurately prepared with its aid, truly orientated and plane surfaced. This instrument is capable of 
achieving all that the new Wright instrument is described as accomplishing, and much more besides. 
The use of a second circle cannot add to rigidity, and the author infinitely prefers the most rigid one-circle 
principle. After 16 years of hard service the author’s cutting and grinding goniometer is as efficient and 
accurate as when first constructed by Messrs. Troughton and Simms. 
O 
