14 PROFESSOR C. V. BOYS ON THE 
The Large Scale. 
The large scale is etched on a piece of plate-glass 9 feet long, 6 inches wide, and 
half an inch thick. The divisions are 50ths of an inch, and there are 4800 of them. 
I made many experiments to find the most suitable kind of scale and thickness of line 
to suit the mirror which I had to use. I shall return to this point when I explain 
the advantages that I have gained by the use of the curious form of beam mirror. It 
is sufficient te state now that the divisions are black upon a clear ground, and that 
the thickness of the lines is greater than at first anyone would be likely to think 
suitable, being about 35 of an inch. The method by which the scale is held rigidly 
aud definitely in place but without strain, is illustrated by the isometrical projection 
in fig. 23, which shows one end only. Z Z are a pair of gun-metal castings screwed 
to the wood frame which is securely wedged into the recess at the east end of the 
vault. wis a brass rod passing through a hole in each casting and able to be 
clamped by a screw at each end. v is a casting with a cylindrical piece turned at 
each end and exactly the same length between shoulders as wis. This rests at each 
end upon a levelling screw, and can be clamped by pinching screws. At the other 
end of the scale, 9 feet away, there is an identical construction. Two plates of glass, 
the back one of which is a dummy, the front one only being divided, rest upon the 
cylindrical projections of v, being definitely held in position by the V notch shown, 
which of course is only at one end. The glass plates rest also against the shoulders 
and against the ends of w uw and are kept in contact by the action of a bent piece of 
brass at each end which lightly presses them towards one another. The glass plates 
are therefore geometrically clamped, each resting on seven points, the one in excess 
of six being introduced to counteract the one degree of freedom which the flexure of 
so long a plate introduces. The middle of the front plate is silvered at the back up 
to near the line of divisions, which is 15 inch from and parallel to the upper edge. 
The levelling screws enable me to bring the upper edge and therefore the line of 
divisions truly level, and this is finally tested by observation with the telescope and 
swinging mirror. The rods ~ and v are then gently worked in or out as needful 
until an observer with his eye at the window of the apparatus in the place of the 
mirror sees the window reflected from the clear glass on the line of divisions. The 
silvering of the middle of the scale is not absolutely necessary, but it enables one 
more quickly to recognize the position of objects placed against the lower part of the 
apparatus and so acts as a finder. When the scale is thus adjusted and has been 
placed with its divisions on a level with the mirror, a division not far from the middle 
will be the point at which a perpendicular dropped from the centre of the instrument 
will cut the scale. The eight pinching screws are then clamped and this division is 
recorded when its position has been more accurately determined by the use of a small 
telescope in the place of the eye. The division 2260 was the perpendicular reading 
