.mi;, j. c. av. huaifrey ox the effects of straix 
•ISO 
When examined under the microscope, the pits at first sight appear to he pro¬ 
jections from the surface, hut by means of the focussing screw it is readily shown 
that they are not projections, but true pits. 
A similar appearance is also seen in the micro-jDhotographs and is apt to be rather 
misleading as to the nature of these geometiical figures, but in all cases they were 
found to be, as stated above, pits descending into the surface. 
The single crystals, when separated one from another, possess all the usual plastic 
properties of an ordinary sample of lead composed of numerous small crystals united 
together. They show no signs of sudden cleavage or parting and may be bent 
double without breaking, and may be hammered out iuto any shape. 
The first observations made on the effects of strain on single crvstals were in 
connection with the formation of slip-lines. The specimens were cut to about 
2 inches to 3 inches long l^y T inch udde by -g- inch thick, and were generally taken 
from a single crystal; if, however, a sufficiently large specimen could not be obtained 
from a single crystal, the piece was cut so that the central portion, where the effects 
of strain were studied, consisted of a crystal extending light across the specimen 
from side to side, as well as through from front to back. After the specimens were 
cut out they were carefully etched until a complete system of geometrical jiits had 
formed upon their surfaces ; they were then washed until all the acid was removed, 
and were (piickly dried. The surface remained bright for a sufficient time to enable 
observations to be made and photographs to be taken, but it gradually became 
taiiiished if left exposed to the air. 
The specimens were strained in tension in the small machine descriiied by EwiXG 
and PiOSENHAiN. This could l)e fixed to the stage of the microscope, so that a 
portion of the specimen was kept under observation during the whole process. As 
the stress was applied the specimen gradually elongated, this elongation being due 
to the numerous small slips along the gliding j^lanes of the crystal. These small 
slips are first visible under the microscope as either bright or dark lines upon the 
surface, according to the illumination of the specimen, just as Ewing and Kosenhain 
first observed them ; but when the amount of strain becomes large the actual steps 
formed can be clearly seen. A point of chief interest in the experiments was 
the relation of the slip-lines to the etched pits upon the surface, that is to say, to the 
crystalline axes. Figs. 7 and 8 aiu photographs taken of surfaces after straining in 
tension. In fig. 7, which is at a magnification of 45 diameters, the etched pits had 
been very slowly produced and are of large size ; they are not puite contiguous, but 
portions of the original surface remain between them. It will be seen that the slip¬ 
lines on one face of a pit are iiarallel to one edge of that face. Again, the illumina¬ 
tion is such that a partially formed hexagonal face shows bright, and it will be seen 
that slip has occurred along planes parallel to this face, so that the small steps also 
appear bright. In fig. 8 (Plate 3) the pits were of considerably smaller size, and 
a higher power (100 diameters) was necessary to show them clearly. As before, the 
