M. DES CLOIZEAUX OX AMBLT GrONITE AND MONTEBRASITE. 
583 
Some good measurements, made on very smooth surfaces of the two former cleavages, 
yielded exactly the angle p m= 105°. For the other incidences I obtained the approxi- 
mate values m fzz=135° to 136°, ^ £=89° to 89° 15'. 
The transparency that suitably chosen fragments acquire when cut into thin plates 
has led me to modify a little the orientation of the plane of the optic axes and of 
their bisectors which I found upon the Hebron specimen. By means of artificial twins 
formed of two plates, each of which has been worked perpendicular to the two 
cleavages p and m, and which were united by their faces p, I have been pig. 5 . 
enabled to satisfy myself that the plane of the optic axes situated in the 
obtuse angle p m— 105° traverses the edge but that it is not quite 
m 
normal to m, as I had at first supposed, since it gives mean angles of 
about 82° with m and 23° with p. By the employment of a similar 
process with an artificial made made from the fragments from Hebron, I 
have obtained numbers that are almost identical with those just given. 
The bisector of the acute angle of the optic axes is negative, and does not seem to be 
p 
exactly parallel to the edge — ; but its exact direction in the plane which contains 
both it and the optic axes cannot be determined with great exactitude on account of 
the difficulty of obtaining plates which are exactly perpendicular to it, and more par- 
ticularly on account of the great separation of the axes even in oil*. 
This separation, which prevents the centre of the coloured rings being ever seen in 
air, is essentially variable, in consequence of the irregularity of the interlacing laminee 
in the interior, the existence of which is evidenced in particular on the surfaces of the 
nacreous cleavages by a waviness or by slight reentering angles visible in a bright light. 
I have found upon some of the plates for the acute angle of the optic axes for red in oil 
between 14° and 20° C., 
2H a . r =95°48'; 99° 39' ; 101°; 102°; 102° 38'. 
The obtuse angle containing the positive bisector gave measurements 
2H o .,=102°50'; 104° 21'; 106° 105 
* In the neighbourhood of the bisector of the obtuse angle of the optic axes difficulties of the same kind present 
themselves. It is, in fact, established by the simultaneous maximum extinction of the light in both parts of a twin 
formed of two plates parallel tom, of which the facesp are brought together in opposite positions, that the plane of the 
p 
axes is parallel to the edge — , which should be a little oblique to the positive bisector, inasmuch as the coloured 
m 
rings seen in oil through the faces m do not seem to be at the same distance from the normal to these faces. 
The position of the bisector itself could not, however, be precisely determined ; for even when plates inclined at 
about 7° or 8° upon to, as nearly as possible perpendicular to the plane of the axes, were used, it was scarcely 
possible to measure with any exactitude in oil the angle that each of them makes with a normal common to 
P 
the plates and to the edge — • Some experiments led to the conclusion that this normal made with the positive 
TO 
bisector an angle of about 4° to 5°. 
4 1 
MDCCCLXXIII. 
