574 
M. A. L. 0. DES CLOIZEAUX ON THE DISPEESION OE THE 
The forms which are most frequently met with on the crystals which I have had at 
my disposal are h\ h 3 , m, g 3 , g 2 , g\ o\p, a\ e\ d\,b\, a 3 . The forms Af, o\ (fig. 7), 
e\ (fig. 8), <p (fig. 9) are, on the contrary, very rare ; e 3 and y have only been announced 
by Dauber (fig. 10). I have supposed the existence of x and b^ corresponding to these 
two last faces by analogy with what I have observed upon the crystals I myself examined, 
on which there occur both in front and in rear planes such as ^9 and a\ o 1 and a\, e 1 and b^, 
d\ and a 3 , e\ and <p, which make respectively almost equal angles with A 1 . The plane 
of the mean optic axes is very nearly parallel to o\ In a made artificially formed 
of two plates cleaved in the direction g l and twinned after the plane A 1 , the maximum 
extinction takes place at the same moment for both plates ; these plates therefore have 
their optic axes situated in planes perpendicular to one another, and each of them 
inclined at 45° to the face of union A 1 . As the yellow colour of the substance does not 
permit of any certain results being obtained with monochromatic glasses of red or blue, 
it is impossible to determine directly the angle which the plane of the axes corresponding 
to the red rays makes with the plane of the axes which correspond to the blue rays ; 
this angle must, however, be very small, in consequence of the weak horizontal and 
twisted dispersion which is exhibited in oil, and to which I have alluded above. The 
bisectrix of the acute angle formed by the optic axes is negative and normal to the 
horizontal diagonal of the base. The separation of these axes varies with the crystals, 
and even with different portions of one and the same crystal. The following are the 
actual results that I obtained from three different crystals, each of which contributed a 
couple of plates normal to the acute and obtuse bisectrices : — 
First crystal cited in my ‘ Manual.’ 
2H a =89° 34', 2H 0 =128° 6', whence 2Y=76° 10', j3=l-67 for the red rays. 
2H a = 90° 54', 2H 0 =127° 6', whence 2Y=77° 2', /3=l-69 for the blue rays. 
Second crystal recently examined. 
2H ffi =85° 41', 2H 0 =139° 3', whence 2Y=71° 56', /3=l-69 for the red rays. 
2H a =86° 12', 2H 0 =138° 32', whence 2Y=72° 18', j3=l-71 for the blue rays. 
Third crystal recently examined. 
2H a =86° 24', 2H 0 =144° 24', whence 2Y=71° 26', >3=1-72 for the red rays. 
2H 0 =87° 30', 2H 0 =144° 8', whence 2 Y= 72° 1', >3=1*74 for the blue rays. 
Plates cut normally to the acute bisectrix in three other crystals gave for the red rays 
III. 
I. II. r ^ , 
2H 0 =141° 17' ; 138° 42'; 140° 32', first portion; 138° 48', second portion. 
The dispersion belonging to the optic axes, weak in oil and in the interior of the 
crystals with §<v is, on the contrary, very considerable in the air ; for on the second 
