THEORY OF THE REFLECTING EXTENSOMETER. 103 
rotating the telescope. This last fine adjustment may be regarded 
as not sensibly altering the indicated condition of perpendicularity 
as between scale and sight-line. In such a case the correction a 
is at once applied to the reading. For the first reading R, say 
will be 0, and to the second R, the correction « must be applied 
so that the corrected value F’ is given by the equation 
R= Ry + 2 
« being the tabular correction for R,. 
But if for any reason it is inconvenient to make the zero thus 
coincident with the plane containing the sight-line and reading- 
wire, then R = R,- R,, and we must employ 
R = (R,-R) + 2 
« being the tabular correction for R = R,—R,. It is perhaps 
hardly necessary to point out that the difference of the tabular 
correction must not be taken in the form «,—«,, that is to say, 
the total correction is not the difference of the corrections for the 
two readings. In the preceding observations it is of course 
assumed that the initial position of the line FG is vertical to the 
test-piece H/’ Fig. 1. 
When two scales are read and give different results, the correc- 
tion « should be for the mean of the differences of their readings, 
see § 11. 
6. Adjustment of the prism perpendicular to the test-piece.—The 
table of corrections in § 4 indicate the necessity of starting the 
measurement of an elongation, or of a compression, with the prism 
in the position of adjustment, whenever very accurate results 
are desired. In Fig. 3,1a small bar BB’ is shewn passing through 
the centre of a small brass cylinder forming a handle for the 
mirror apparatus, its length being 30 mm. On releasing the small 
Screw J in this brass handle, the bar and handle may be rotated 
with respect to the line j joining the knife-edges of the prism—i.e., 
the line FG—and consequently may in this way be placed exactly 
at right angles to the knife-edges. This is conveniently effected 
CON a OP ROO 
1 Fig. 3 in Professor Warren’s papers 
