474 PROFESSOR HENRY A. MIERS : AN ENQUIRY INTO 
face all belong to the same form, it is easy to calculate the theoretical readings for 
the true octahedron faces; they would be 
o = 328° 2', 
OJ = 257° 31', 
o' - 148° 3', 
J = 77° 31'. 
so that 
-ooj = 70° 31', 
..o'oj' = 70° 32', 
The angle (a^ = = ya) of the triakis-octahedron replacing each octahedron face 
is also easily calculated, and is for the form replacing o 0° 5', 
^ 0°9l', 
. o' 0° 61' 
'4 ’ 
CO U 8 j . 
This example indicates that— 
(1) The octahedron angle of the crystal is really the true octahedron angle 
(70° 31'-70° 32') ; 
(2) That there is a different flat triakis-octahedron replacing each face, although 
they only differ to a slight extent. 
In this instance the images, a, j3, y, of each set seem to iDelong to the same form, 
for the a and ^ images are in the same vertical plane ; but this is not always the 
case ; the same crystal, measured on November 28, gave the following readings from 
o and CO [cf. figs. 11 A, 11b) :— 
“ = 290° 584', 
o. = 290° 55', 
= 290° 52f', 
= 220° 291', - 
co^ = 220 ° 22 ', 
_c,j, = 220° 201/. 
Whereas from co' co'^ = 40° 30', co'^ = = 40 21'. 
Hence the reading for the true octahedron face co' is 40° 24'; therefore the reading 
for CO is 220° 24', and the three faces replacing co are unequally inclined to w, and do 
not belong to the same form (fig. 11a). 
If it be true that the vicinal faces, whether they belong to the same or to different 
forms, are always triakis-octahedron faces, then of the faces replacing an octahedion 
plane which should yield an image at o one is always situated on the hoiizontal 
