248 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE _ [VOL. XI 
to expect that these would be found to be enclosed by ordinary crystal 
faces, and that reflections from them would be clear and distinct. In 
this case it would be expected that the reflections would be thirteen in 
number, a central one representing the unetched crystal face; two on 
each side in a direction normal to the vertical axis of the crystal, repre- 
senting the long faces and four on each side of a line connecting these, 
forming roughly, if joined, two crossing lines, representing the smaller 
faces. 
Instead of this, the reflections took the form of a bright band, bright- 
est in the centre, from which point it steadily became fainter, until at 
some distance at each side there was a break, followed by a hazy spot at 
the end. There also extended from the centre two fainter lines above 
and two below in a slanting direction. This figure would represent the 
reflections from a boat-shaped pit whose sides and bottom are alike 
curved. Subsequent investigation however tended to show that this 
curvature was only apparent and that the effect was due to the fact 
that the figure faces are numerous, small and differing but slightly in 
their inclination to the axes. 
An additional exposure of the same crystal, bringing the total up 
to 92 minutes, under the same conditions, gave pits which were more 
distinct and considerably larger. When set up on the goniometer as 
before and examined, it was seen that while the reflection was, as in 
the previous experiment, in the form of hazy bands, the central one 
had become noticeably shorter, and all through it were scattered more 
or less distinct spots of light, which would indicate that for the first 
time some of the figure faces were large enough or flat enough to yield 
definite reflections. However they were rather numerous and close 
together, and consequently could not all of them represent possible 
crystal faces. 
Further exposures gave results of steadily increasing clearness. 
After the total had been 133 minutes several of the reflections were the 
best so far observed. This improvement continued on additional 
application of the corrosive, until the total time was 23} minutes, after 
which there was no increase in the distinctness. Corrosion was dis- 
continued in the case of this crystal after 333 minutes, but other crystals 
were etched under the same conditions for a very much longer time. 
A white diopside from Zillerthal in fifteen minutes, gave small and 
numerous pits on the orthopinacoid. After further action for fifteen 
minutes, the pits were sufficiently well developed to give clear reflec- 
tions, but after a total of forty-five the point of greatest perfection had 
been passed and the larger, shallow pits were no longer definite enough 
to be read on the goniometer. Another specimen, a green diopside from 
