398 
DR. A. E. H. TUTTON ON THE 
of the determination of these indices therewith before opacity supervened (which 
occurred within a further hour). The values obtained from this ground prism proved 
to confirm fully the results previously obtained, and as the third (intermediate) index 
of refraction had been afforded twice over absolutely directly by two natural prisms 
employed, the whole of the optical constants were thus ascertained in a manner which 
rendered them fully trustworthy. 
Morphology. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes .— 
a: b : c — 07490 : 1 : 0'5044. Tops0e’s approximate values 07407 : 1 : 0'5007. 
Axial Angle. — f3 = 103° 50'. Approximate value of Tops0e, 104° 15'. 
Form,s Observed.—a {100}, b {010}, c {001}, p {110}, p' {120}, q {Oil}, A {201}, 
o' {ill}, and r! {l2l}. The forms a {100} and g/ {120} were not observed by Tops0E. 
Habit. — More or less tabular parallel to c {001}, fig. 1 , to short prismatic parallel 
to p {110}, occasionally with especial elongation of one parallel pair of the p-prism 
faces only (fig. 2). 
The great majority of the crystals obtained were of the tabular type illustrated in 
fig. 1, and this type proved very useful in the optical work, as the bisectrix of the 
obtuse angle between the optic axes is nearly identical with the normal to the plate, 
which latter is parallel to the faces of the basal plane c {001}. The type shown in 
fig. 2 was represented by one of the finest and largest crystals obtained. In all the 
crystals the c-faces and p-faces predominated, usually the former to the greatest 
extent. The g-faces were, as with all the potassium salts of the series, small. The 
r'-faces were generally rather large, and sometimes were very prominent indeed, a fact 
which has been observed to be characteristic of the iron groups of both the double 
sulphates and double selenates. Three good measurements were obtained from little 
a-faces, of a narrow type but excellently reflecting. The faces of h {010}, o'{Ill}, 
and n r {l21} were only very minute (usually almost imperceptible lines), and afforded 
no adequate reflections of the signal. Several little g/-faces were observed, however, 
and in two cases good measurements were obtained. 
The table of angles is given on the next page, and it is a somewhat surprisingly 
satisfactory one, considering the circumstances under which these six crystals had to 
be measured. 
