102 G. H. KNIBBS. 
is negative. For this case the values of the corrections require 
very small alterations which amount to 
i. 3 t san? w R 2828 
2 c 
7 See op sve vans (8) 
These very small quantities are given in the following table. They 
are to be added with their attached signs to the corrections in 
Table I. 
OS @ 
TasxeE II. 
Secondary corrections to be applied when the mirror is on the 
side of the test piece remote from the telescope. 
Scale-reading in 
millimetres 
ingen mm. 0 +01 #02 +03 +05 +07 2:09 4°12 
150 200 250 300 350 400 450 500 
A similar table with the argument L = 2270 = 500/ may also 
be required, as it may be sometimes convenient to use the instru- 
ment at that distance. It may be readily formed with sufficient 
precision from the preceding table by multiplying the upper five 
horizontal lines by 2 throughout, with the one exception of the 
angles 20, These must be left as they stand. This simple method 
of obtaining the corrections for the other length depends upon the 
fact that the term + 3/ tan?/4Z is really insensible throughout 
for the values of » in question. 
This latter distance, viz., 2270 mm. is about the maximum at 
which the scale may be placed from the mirror, forasmach as at 
greater distances it would be beyond the focal limit of the telescope. 
5. Application of scale-reading correction.—In order to readily 
obtain accurate results it is essential that the face of the scale 
should be at right angles to the sight-line of the telescope. Then 
if the zero of the scale be placed in the plane containing both this 
line, and as the case may require, either the vertical or else the 
horizontal diaphragm wire used in reading the scale, the mirror 
attached to the prism may be rotated until the zero of the scale 
seen by reflection therein becomes approximately coincident with 
the cross-wire; when the final adjustment to the zero exactly, 
may be conveniently made by means of the slow motion screw — 
