568 
M. A. L. O. DES CLOIZEAUX ON THE DISPERSION OE THE 
Calculated angles. 
' *m m 120° 1' 
ffl/t 1 150° O’ 30" 
mg 1 119° 59' 30" 
j h\h l 171° 47' 
h^g 1 98° 13' 
h\b> 167° 0' 
m hf 163° 0' 
l h>g l 90° 
r po l 144° 18' 
j o'/i 1 160° 32' 
! *ph l 124° 50' 
| o 1 t o 71° 24' over A 1 
| h\y adj. HO 0 20' 
L h 1 x y 69° 40' over p 
f pm 119° 39' 
I mm adj. 120° 42' 
m m 89° 23' on g l 
*p a 1 90° 
p e 1 134° 42' 
Instead of the prism hf of Phillips, I have observed the prism hf on many crystals of 
Morvenite, and instead of o', given by Greg, I have found of- with 
Calculated. Measured. 
pof 131° 49' 131° 30' 
of 173° 1' 
There exist two principal cleavages, the one easy and rather neat parallel to the plane 
of symmetry g\ the other less easy parallel to the base p. 
The plane of the optic axes and their acute positive bisectrix are perpendicular to the 
plane of symmetry. The disposition of the coloured rings in plates normal to the two 
bisectrices had heretofore induced me to look on the mean optic axes as situated exactly 
in the plane bisecting the angle p h 1 ; but I have since assured myself that this was not 
altogether true. In fact the results from a great number of observations made at 15° C. 
by means of two Nicol prisms crossed at right angles in the direction in which the maxi- 
mum of extinction takes place in two neighbouring sectors, taken from many plates of 
Morvenite parallel to the plane of symmetry, may be stated as — 
First. That the plane of the axes corresponding to red rays makes angles of about 
25° 42' with a normal to p, 
29° 28' with a normal to h\ 
Second. That the plane of the axes corresponding with the blue rays makes an angle 
of about 
25° 5' with a normal to p, 
30° 5' with a normal to h 1 . 
a Phil. = P hillips. b Dx.=Des Cloizeatjx. 
Measured angles. 
120° 1' (Kohler;. 
150° 0' 24", mean (Dx.) ; 149° 32' (Phil.) a . 
120° 15', mean (Dx.) ; 120° 56' (Phil.). 
171° 4' (Phil.). 
166° 30' (Dx.) b . 
162° 50' to 163° (Dx.). 
124° 50' just; 124° 53', mean (Dx.) ; 125° 5' (Phil.). 
110° 28', mean (Dx.) ; 110° 26' (Phil.) ; lll°15'(K6hler). 
69° 39', mean (Dx.). 
119° 5' approx. (Dx.). 
120° 45' to 121° 37' (Dx.) ; 121° 6' (Levy) ; 121° 28' (Kohler). 
89° 2', mean (Dx.). 
Twin plane of the simple macles. 
Face nearly corresponding to the twin plane of the crossed macles. 
