( 73f^ ) 



pounds of trivalent titanium and vanadium we investigated some 

 without, liowever, having found anything noteworthy as yet. The 

 selective properties in tiiis series culminate for chromium ; we shall 

 therefore restrict ourselves to a closer ijivestigation of some chromic 

 compounds already discussed in our former paper. 



Chromium aluut. 



From the well-known regular crystals plates of a thickness of about 2 

 aiid 3 m.m. were cut. At 18" a rather intense band 669,8 — 671,6 is seen 

 in the I'cd; at — 198° it becomes considerably narrower, viz.: 668,6 

 — 669,4, the centi-e shifting 1.7 (in towards the violet; moreover 

 another rather strong line 670,2 appears ; between 619 and 716 no 

 less than 21 fainter and sharper bands and lines are actually visible. 



In a tield of 34 kilogauss the two principal lines appeared broken ; 

 the horizontal distance of the corresponding edges of their upper and 

 lower halves, henceforth bi-iefly called the break, amounted to about 

 0,10 (ill; the sense was opposite'^). Band 668.6- 669.4 shows one tine 

 narrow satellite oji the red side, towards the violet two of them; 

 the former disappeared in the Held; the two latter ones became very 

 vague, and seemed, as seen w^ith sunlight, to join in the break of 

 the principal band. 



Ruby. 



^ 15. With the square plate (7X^X3 m.m.) mentioned in our 

 preceding paper a long edge contained the optical axis. From the 

 same ruby cone a small quadratic prism (1,5 X 'J^^ X 4 m.m.) was 

 now ground, the axis being parallel to a short edge. With the slight 

 thickness of 1,5 m.m. sufficient absorption is shown even with grating 

 dispersion. We must now distinguish the cases that the optical axis 

 is II or ± with respect to the direction of the field. 

 I. Optical axis || direction of field: 

 A. Pair of bands in the blue at — 193°. Besides the two bands 

 in the red already described, a pair in the blue are rather striking 

 among the other 8; we shall briefly call these ^^ and 7),. At — 193° 

 their situation is: 7i, = 474,2— 474,9, and j5, = 476,1— 476,5 (at 

 18° they lie 474,9—475,7 and 476,5—477,1, more towards the red). 

 The distance of the central lines measured in the grating spectrum, 

 amounted to 1,63 fif/. In a tield of 36 kgs (== kilogauss) the break 

 for B^ amounted to 0,04 ft [i and for B^ to 0.055 mi, the sense being 



') I. e. with respect to that wliich lias been round up to now for all vapours: 

 such an opposite sense was also obseived by J. Becquerel in most cases. 



