MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
403 
reflected from the mount-plate, and they were readily brought by slight rotation of 
the circle and plate to coincidence with the spider-lines seen directly, any slight 
vertical displacement being readily corrected with the aid of the appropriate (back to 
front) movable segment of the crystal-adjusting apparatus. When the adjustment 
to coincidence was perfect the reading of the circle was taken, this reading then 
corresponding to the identity of the normal to the plate and to the crystal face 
c{00l} with the axis of the polariscope. The cell containing the immersion liquid, 
cedar oil, was then raised into position so that the crystal occupied its centre. The 
crystal plate was next adjusted, by slight rotation in its own plane, using white light, 
so that the optic axial plane b {010}, the symmetry plane, was strictly horizontal if 
not already so adjusted. This was effected by viewing and adjusting the interference 
figure in convergent polarised light (the Nicols being crossed at 45° and 135°); the 
correct position was attained when the optic axial “ eyes ” (the very small innermost 
rings and sharp vertices of the hyperbolic brushes) to the right and left of the normal 
position, as the circle was rotated, first one way and then the other, moved exactly along 
the horizontal cross-spider-line, by which they were bisected. The conditions were 
then attained for the measurement of the optic axial angle for the usual series of six 
wave-lengths of light, the crystal plate remaining locked to the circle throughout the 
measurements. These measurements were then carried out with the aid of the 
spectroscopic monochromatic illuminator, the vertices of the optic “ eyes ” being 
brought to the centre of the pair of vertical spider-lines for each wave-length in 
turn. Not only were readings for each of the two “ eyes” to right and left of the 
normal taken, but also the readings for 
the other two optic “ eyes ” observed on 
rotating through the whole circle. By 
this means duplicate readings 180° apart 
were obtained for both the obtuse and 
acute true optic axial angles 2V 0 and 2V a , 
the plate being immersed in a liquid of 
similar index of refraction and conse¬ 
quently affording not merely an apparent 
angle in oil but the real optic axial angle 
within the crystal. For as the plate was 
rotated through a complete circle four 
optic eyes were met with in succession, 
each at its correct position as within the 
crystal. The mean (half-way) positions 
90° apart for any one colour of light were those of the two median lines, perpendicular 
to each other, the axes a and y of the optical indicatrix. The procedure will be rendered 
clear from the following record of the observations with Plate 1 ; fig. 3 will also assist in 
elucidating the arrangements, A representing the crystal and B the glass mount-plate, 
VOL. CCXVIII.-A. 3 H 
S 
iQcr xi$ 
