94 W. H. WARREN. 
hundred and fiftieth part of an inch; there is no difficulty in sub- 
dividing the divisions on the dial if desired, in order to read . 
xis mm. The writer has just used one of the instruments In 
determining the elastic deflections of some rails for Western 
Australia, and he proposes to use it in connection with a series of 
tests of brickwork and concrete columns. 
Tue THEORY or tue REFLECTING EXTENSOMETER 
or Pror. MARTENS. 
By G. H. Kyrpps, F.R.A.8., 
Lecturer in Surveying, University of Sydney. 
[Read before the Royal Society of N. 8. Wales, August 4, 1897.] 
once theory sometimes inadequate. 
Descri of the extensometer. 
Relation peeiii extension and scale-reading. 
Construction of tables of corrections to scale-reading. 
Application of scale-reading correction 
Adjustment of prism perpendicular to tesbplese. 
Examination of the pivot axis of the mirror. 
Parallelism of the rotation axis of the mirror with the knife- 
edges of the prism. 
9. Error due to longitudinal movement of the test-piece. 
10, Error from rotational movement of the test-piece. 
11. Disposition of the apparatus in testing, and general. 
1. Approximate theory sometimes inmadequate.—The theory of 
the measurement of very small extensions by means of Professor 
Martens’ reflecting extensometer, which was exhibited and des 
cribed by Professor Warren at the last meeting of the Royal 
Society, leaves little to be desired, when the extensions do not 
exceed the limits contemplated in that description, that is, when 
they are extremely small as compared with the distances betweet 
the knife-edges of the rotated prism carrying the mirror. And 
a2 ere er 
