258 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE  [VOL. XI 
hydrofluoric acid for 15 minutes was mounted on the goniometer and 
readings were made at intervals along each one of the light-paths 
Although they were frequently quite dim, they could still be traced in 
an unbroken series around the crystal in several directions, as is shown 
in the accompanying stereographic projection. This is characteristic 
of all the zones. 
These observations show very clearly that the faces which form the 
sides of a pit, and which on microscopic examination resemble crystal 
faces so closely, are in reality very different. In no case do the para- 
meters bear a simple ratio to one another. In fact, the only type of 
crystal face with which they seem to have anything in common is the 
vicinal face. 
The length and the continuity of the light-paths would seem to 
indicate that the reflexions are those from curved faces, and this has 
been generally assumed. But there are two suggestive observations 
which militate against this assumption. In the first place, scattered 
among these indistinct rays there are to be found faint signals like 
those which might be expected from very small faces. Again it was 
frequently observed that in order to trace the light-paths it is necessary 
to move the crystal so that various parts of each face come successively 
into the field of vision. Further, it was also found that by narrowing 
the aperture of the telescope’s eye piece, the light-paths become short- 
ened and replaced by a series of hazy spots, and sometimes change into 
clear, though faint crosses. 
Evidently then the continuity of the light-paths is not due to the 
curvature of the figure-faces, but to the fusion of reflexions from numerous 
pits. This leads to the rather unexpected deduction, which is, however, 
borne out by numerous observations, that the corresponding figure- 
faces of pits on the same crystal faces are not identical, though their 
difference is slight. 
Although we must not look upon pits as being enclosed by true 
crystal faces, it is evident that their bounding planes represent as close 
an approximation as possible under the circumstances. Goldschmidt 
has pointed out that in pit formation there are two opposing tendencies, 
one of which would result in the formation of round excavations if 
unopposed, and the other in the development of true crystal faces. 
Thus the net result is these faces which are approximately regular, and 
hence are of value in the determination of the physical properties of 
crystals. 
Since the symmetry of any pit is an expression of that of the crystal 
face upon which it is developed, it is natural to expect that there would 
be some relationship between the pits on various faces of the same 
