LIV, DISCUSSION. 
Probably the most perfect apparatus for producing this part of 
the diagram would consist of mirrors attached to the test piece, 
whose angular displacements were proportional to the deforma- 
tions, and the light from which was focussed by means of a camera 
with the sensitised plate made to slide at a rate proportional to 
the stress on the test piece. Such an instrument could easily be 
constructed, and its advantages over every other so far construc- 
ted were obvious. Professor Johnson proposed, however, to locate 
a point on the ordinary small scale autographic diagram which he 
had defined as the relative elastic limit, thus adding another kind 
of elastic limit to the three referred to by Mr. Haycroft. Such 
autographic diagrams are exceedingly useful and instructive, and 
should be drawn for every test which is continued to the point of 
failure, but it appeared to the author undesirable to push the 
data so obtained too far. The yield-point was perfectly well 
defined on such diagrams for ordinary ductile materials, and it 
was usual, although incorrect, to record this point as the elastic 
limit. The author proposed to record the yield-popint, as the yield- 
point or commencement of the large plastic deformations, as he 
considered the practice of recording it as the elastic limit is mis- 
leading, also that it was preferable to any arbitrary point such as 
Professor Johnson’s, which lay somewhere between it and the 
elastic limit, which latter is very imperfectly defined on an ordi- 
nary autographic diagram. If the elastic limit were desired—and 
this should always be obtained in an important test—then it was 
imperative that an accurate extensometer be used, such as the one 
exhibited by the author at the June meeting of the Society— 
“The Martens mirror apparatus.” It was absurd to say that fine 
measurements were useless for such a purpose, and that rough 
ones are preferable. None of the methods described by Professor 
Johnson, in conjunction with American tests, were as exact as the 
records produced by Professor Martens’ apparatus. 
The subject of impact tests have only been referred to by Mr. 
Shaw, who explained a very ingenious application of the use of 
copper cylinders in connection with such tests. 
