570 
M. A. L 0. DES CLOIZEAUX ON THE DISPERSION OE THE 
a considerable delay between the moment when certain optical phenomena are mani- 
fested, and the moment of the change of temperature. This delay, produced by causes 
probably very complex, is all the more remarkable from the plate on which I made most 
of my experiments having dimensions no greater than two millimetres in width by 
half a millimetre in thickness, dimensions far less than those of the plates which I am 
in the habit of introducing into my air-bath, and of which the modifications, thermic as 
well as optical, have always presented themselves to me as nearly as possible simulta- 
neously. 
Wohlerite. 
In my ‘ Manual of Mineralogy’ I described the crystals of Wohlerite as derivable 
from a prism of very nearly 90°. In the point of view from which I had been induced 
to look at them, from a consideration of the orientation of their optic axes, they appeared 
to offer a certain number of homohedral forms, associated with forms that were hemi- 
hedral or hemimorphic, analogous to those that 1 have drawn attention to in Harmo- 
tome. Since I have established that this last mineral belonged in reality to the clino- 
rhombic system, I have sought to ascertain whether this was not so also in the case of 
Wohlerite, all the forms of which would in that event be homohedral. But in this case 
a study of the different varieties of dispersion is rendered difficult by the yellow colour, 
and by the imperfect transparency presented by the substance even when in very thin 
plates. Besides this, contrary to what one finds in the case of Harmotome, whilst the 
dispersion belonging to the optic axes is very distinct, the horizontal and twisted disper- 
sions, which should be sought for in the plates normal to the two bisectrices, are, on the 
contrary, but slightly evident. However, on examining in oil some thin plates placed so 
as to have the plane of their optic axes horizontal and perpendicular to the plane of 
polarization, I observed in the plates normal to the obtuse positive bisectrix, some faint 
blue and red fringes disposed in contrary directions above and below the bars which 
traverse the two annular systems. These colours indicate, then, the existence of an 
appreciable twisted dispersion. In the plates normal to the acute negative bisectrix, 
the transverse bar of each system is bordered on one side by very pale blue, and on the 
opposite side by an equally pale yellow. The horizontal dispersion is thus feebly indicated. 
Furthermore, in the former, as in the latter plates, the edges of one system of rings often 
show a colour decidedly more brilliant than those of the other system ; but this apparent 
dissymmetry is only the result of the imperfection of the plates, and especially of the 
unequal transparence presented by their different parts in consequence of internal fissures. 
The crystals of Wohlerite ought, then, to be referred to an oblique rhombic prism, in 
which the plane of symmetry is normal to the plane which contains their optic axes. 
The primitive form which it seems most convenient to choose is a prism with an angle 
of very nearly 90°, which presents a cleavage, easy though interrupted, parallel to its plane 
of symmetry, and cleavages which are more difficult in the directions of the lateral faces 
m and of the plane h\ which is parallel to the horizontal diagonals of the base. 
The figures 1 to 9 (Plate XXXIV.) represent the actual appearance of the crystals 
