THE VARIATION OF ANGLPIS OBSERVED IN CRYSTALS. 
4H5 
After further readjustment :— 
At 4.45 P.M. ; t = 17°. Approximate readings for true 
culie faces are therefore 
^ a . 
93° 
20 '; 
imao’e 
11 ' 
10 " 
above 
cross-wire. 
A r 
= 93° 
25i' 
A^ 
93° 
25'; 
5 ? 
7' 
10 " 
below 
5 5 
B'. 
3° 
31'; 
?? 
. 
. 
on tiie 
5 5 
B' = 
= 3° 
29f 
B', 
3° 
28'; 
55 
• 
• 
55 
55 
A'. 
273° 
20 '; 
55 
• 
• 
55 
55 
A'^ 
273° 
19'; 
55 
0' 
0 
20 " 
above 
55 
A' = 
= 273° 
221 ', 
B. 
183° 
31'; 
5 5 
4' 
10 " 
below 
55 
_B, 
183° 
28'; 
55 
1 ' 
50" 
above 
5 5 
B = 
= 183° 
291. 
The crystal was re-immersed at 9.45 p.m. ; t = 18°. At 10.15 p.m. ; / = 18° ; the 
imao’es from B indicated a re-entrant anode. 
o o 
“B, 183° 40'. 
B^ 182° 58'. 
A„ 93° 31' . . . A„ two images; one 10 ' below, the other 14' 40" above 
A 0^*^ 11' A 
OO L ~± , . . 5 5 5 5 5 5 55 55 55 
B = 183° 19'; A = 93° 22 ^'; and AB = 89° 504'. 
Tills example illustrates the difficulty of adjusting correctly a crystal which is lialile 
to the growth of vicinal faces, and the impossibility of measuring accurately the 
angles between the faces of the simjile forms which are replaced liy them, excejit by 
tracing these changes. In sodium chlorate the vicinal faces sometimes lielong to the 
form {hhO], and they make angles of from 3 to 10 minutes with the ideal cube face ; they 
sometimes make re-entrant angles with each other, but the dlfficidtles of adjustment 
and measurement are increased by the fact that the vicinal faces often belong to 
various forms [hJd], and are unequally inclined to the cube faces. The same general 
features hold as for alum : the vicinal forms usually become more acute as the growth 
proceeds; and they are less acute for the parts of the crystal which are most deeply 
submerged. A cube face of the crystal just described, for example, gave on May 27, 
1899, two images separated by 2g minutes when at a depth of about 1 inch, and two 
images separated by 12 minutes near the surface of the solution. 
Zinc Sulphate and Magnesium Sulphate. 
That tlie same features are also exhibited by crystals l^elonging to other systems 
than the cubic, was proved by experiments on these two substances. 
