Chief dimensions of the crystal 

 structunj. 

 X : J/; : (o = 4,5004 : 7,6225 : 7,0593. 



Optical orientation : 



The axial plane for violet, blue 

 and green is |001| but for the 

 orange and red rays it is, however 

 |0J0|. The first diameter for all 

 colours is the rx-axis of— charac- 

 ter. The axial angle for violet is 

 nearly 0'^. 



( 793 ) 



Chief dimensions of the 

 crystal structure / : *// : to = 

 = 4,7327:8,0270:7,2267. 



Optical orientation : 

 For all colours the axial 



plane jOOlj. 



The tirst bisectrix is 



the a-axis of -|- character. 



Weak dispersion : q > v. 



The apparent axial angle 



in cedar-oil is about 50°. 



Chief dimensions of the 

 crystal structure / : i^ : w = 

 4,7883 : 8,3061 : 7,3077. 



Optical orientation : 

 For all colours the axial 

 plane is |001| with the 

 <2-axis as first diameter of 

 -[- character. 



Middlemost dispersion : 



The apparent axial angle 

 in cedar-oil is about 70°. 



It should, however, be remarked that this Tri-p-Chlorotriphenyl- 

 methane exhibits also a very interesting optical variability as will be 

 noticed from the subjoined observations : 



a. Crystals from 0. Fischer ; the compound is recry stal Used 

 from petroleum -ether (b.p. 40° — 60°). 



For all colours the optical axial plane was: |010|. Very strong 

 dispersion : (? > i' ; the «-axis, is the 1«^ diameter and possesses a 

 negative character. The apparent axial angle in olive-oil is very 

 small and amounts to about 5^. 



With other crystals, particularly the thicker prisms, I found that 

 the axial plane for violet and blue rays is {001 1 but for all other 

 colours |OiO|; the «-axis, is the first bisectrix, but now of a positive 

 character; the very strong dispersion was: 9 > green. 



Other little crystals only exhibited the violet in |001|, and the blue, 

 green, red, yellow etc. in the plane jOlOj. 



b. Crystals from the collection of the Organic Chemical Ijaboratory 

 at Leiden, prepared by P. J. Montagne. They loere optically perfectly 

 identical with the crystals which I examined previously. ') Some of 

 the crystals had become opaque but had retained their form. This 

 fact is already mentioned by Montagne =) who observes also that the 

 meltingpoint remains practically unaltered. 



At my request Dr. Montagne forwarded me some powder of Tri-p- 

 Chlorotriphenylmethane from tetrachlorobenzopinacoline, which after 

 recrystallisation from petroleum ether showed the following properties : 



The axial plane for all colours is now |001|. Very strong disper- 

 sion: Q^v , the a-axis is the 1st bisectrix; the ai^parent axial angle 



1) Zie Receuil d. Trav. d. Ghim. d. Pays-Bas, 24. 124, 131. (1905). 

 2. loco cit. p. 122. 



