18 PROFESSOR C. V. BOYS ON THE 
the true optical distance from the reflecting surface of the mirror to the foot of the 
perpendicular upon the scale. This includes the small correction for the reduction in 
distance owing to the refractive power of the glass composing the mirror and front 
window. The divisions on the scale are, of course, placed on the side facing the 
instrument, so that no refractive correction is needed for the scale. 
The Optical Compass. 
In order to make the horizontal measures of the distances between the wires from 
which MM hang, and the quartz fibres which carry mm, measures which have to be 
made with the greatest possible accuracy, I had to design a special instrument which 
was suggested to me by Professor CLIFToNn’s optical compass. That is an arrangement 
by which two microscopes can be made to slide parallel to one another. After being 
simultaneously focussed on the two marks whose distance asunder is required, the 
frame to which they are clamped is rotated so as to bring them relatively unchanged 
in position to view a scale divided by lines microscopically fine. In this way the distance 
is directly transferred to a scale in terms of which it is known. In my case the chief 
difficulty was to keep the whole apparatus confined within the horizontal limit of 14 
inches, which was all I had liked to allow myself in the design of the apparatus itself. 
Into this space I had to get (1) a rotating slide to move on the lid round the axis of 
the apparatus ; (2) a focussing slide to move to and from the plane of the wires and 
fibres ; (3) a pair of traversing slides, each to carry one microscope capable of being 
separated by a fine adjustment and with a motion parallel to the planes of the fibres 
and wires. It was essential, moreover, that the slides should be very rigid, and that 
the focussing slide in its traverse should remain upon the same supports to avoid 
difference in flexure in case there should be any. The geometrical principle was of 
course followed, each moving piece resting on five independent small surfaces, and free 
from mechanical constraint. This instrument is shown in figs. 24 to 29. To avoid 
confusion, the rotating and focussing slides, with scale and micrometer screw only, are 
shown in figs. 24, 25, 26, in full lines upon the lid in chain lines, and the focussing and 
traversing slides in figs. 27, 28,29. The rotating slide R rests upon the lid by 
means of two curved V’s, v,v,, resting in a circular V groove upon the lid, and by 
the flat surface f bridging the V groove at the back. It can, therefore, rotate upon 
the ld without shake, but no other motion is possible. This piece is made very stiff 
by the raised rib x round the triangular part, and by the overhanging ledge which 
extends over its whole width. The rotating slide also carries a micrometer screw 8 
of 100 threads to the inch, with a head divided into 100 parts, and two parallelizing 
screws, 8,S,. On the flat surface before 8,S8,, a glass microscopically divided scale 
stands upon two little glass feet. Full particulars of this will be given after the 
description of the optical compass. It merely rests against the parallelizing screws 
and can be moved bodily to the right by the micrometer screw. No slide of any sort 
