20 PROFESSOR C. V. BOYS ON THE 
the use of focussing collars C, C, which slide stiffly on the microscopes and are so 
adjusted that when the two microscopes are alternately placed in the same groove 
and pushed up to their focussing collars they will each be in focus upon the same 
object. The positions of the grooves are such that the microscopes, when in their 
symmetrical positions, can be brought upon points distant from one another by 1, 4, 
or 6 inches, with a stnall margin on either side of a few hundredths allowed by the 
screw cone. Each microscope is furnished with a cross-wire and an eyepiece divided 
scale, one or other of which can be used according to the position of the positive eye- 
piece. If the microscopes are laid in grooves that do not correspond with one another, 
they may also be focussed upon points 23, 34,.and 5 inches apart. If the two 
traversing slides are made to exchange places, for which purpose the screw cone has 
an extra nut and bearings provided, then distances of 2, 3, and 44 inches can be 
measured also, should any of them be required. 
Beyond stating now that the optical compass does a great deal more in the investi- 
gation than merely measure horizontal distances between vertical wires or fibres, and 
that the geometrical and rigid construction makes it possible to work to the full limit 
which optical definition imposes, I shall not at present explain the details and the 
order of the operations carried out by its aid. They will come more conveniently 
under the description of the experiment itself (Operation 9, p. 40). 
The Small Glass Scale. 
This was made for the optical compass by Zerss. A strip of plate glass 64 x 1 x 4 
inch was divided by lines microscopically fine as follows. <A line was ruled at every 
inch from 0 to 6,and at 2°50 and at 3°50 inches. In addition to these, five lines, 
too Of an inch apart, were ruled on either side of the divisions 1:00, 2°50, 
3°00, 3°50, 5°00, and 6:00. The five on either side of the zero were by inadvertence 
omitted, and the zero line was, by some obscure accident, ruled at ‘04 instead of at 
its true place. This, however, was of no consequence, as the 6-inch distance was 
measured by reference to the divisions ‘04 and 6°03, 6°04, or 6°05. The 4-inch 
distance (not yet wanted, however), by reference to 1:00 and 5:00, and one or two 
contiguous divisions, and the small distance which was to have been 1 inch about, 
but which is in reality almost exactly ‘9 inch by reference to 2°55 and 3°45. When 
I was in Cardiff at the meeting of the British Association, Professor VirIAMU JONES 
allowed me to measure the absolute distances between these divisions and a few on 
either side upon his measuring machine. This machine is one of WHITWORTH’s ten- 
thousandth machines, but of more than usual stability, and with a bed long enough 
to take in bars three feet long. It is provided with a set of these bars increasing by 
inches from 1 to 12 inches, and with a 2-foot and a 3-foot bar. The glass scale was 
attached to the upper surface of the tail headstock by being simply pressed down 
edgeways upon two wafers of soft wax, and it was pressed endways against a third 
