164 CAPTAIN A. R. CLARKE ON THE COMPARISON OF ENGLISH AND FOREIGN 
and deep foundations of brickwork ; their upper surfaces are about 4 feet 6 inches above 
the floor of the room. Each stone is so cut as to present along its front horizontal edge 
a projecting ledge, the upper surface in fact projecting 3 inches beyond the lower part 
or face of the stone; the vertical depth of the projecting ledge is 3 inches. 
Immediately in front of and close to the piers is placed a large mahogany beam, 
measuring 14 feet in length by 14 inches in breadth, and 9 inches in vertical depth. 
Its position is horizontal and parallel to the length of the room, and to the faces of the 
stone piers ; to its upper surface (which is about 2 feet above the floor of the room) are 
fastened a pair of planed cast-iron rails, 11 inches from centre to centre, and extending 
the whole length of the beam. In the fixing of these rails to the beam, provision 
is made for any warping which the beam might undergo ; so that the rails can always 
be kept straight and parallel. 
This beam being intended to support the standard bars when under the microscopes, 
is not itself supported by the floor of the room, but has, like the stone piers, its own 
foundations. The flooring upon which the observer stands has no contact either with 
the stone piers, or with the foundations by which the beam is supported. Further, the 
foundations for the beam are entirely disconnected with the stone piers, thus (and it 
has been repeatedly and severely tested) no movement of the observer can disturb either 
the microscopes or the bars under observation. Before perfect immunity from disturb- 
ance, however, was obtained, it was found necessary to disconnect the wooden flooring 
entirely from the walls of the room ; the flooring is framed in three separate pieces, 
each being supported by, or simply resting on, four large blocks of india-rubber. 
Micrometer Microscopes — The magnifying power of the microscopes is about sixty. 
The length of the tube from the diaphragm to the object-glass is 12 inches, and from 
the object-glass to its external focus 3 inches. The value of one division of the micro- 
meter is about the 35,000th part of an inch. Each microscope is held immediately in 
a strong hollow gun-metal cylinder about 6 inches in length, the axis of which coincides 
with that of the microscope. At either extremity this cylinder is internally provided 
with circular Y’s, into or against which the tube of the microscope is pushed by springs, 
the tube having two strong accurately turned collars for this purpose at one-fourth and 
three-fourths of its length. The upper collar has a flange which determines longitudi- 
nally the position of the microscope with respect to the gun-metal cylinder ; while at 
the same time the microscope is free to revolve in the cylinder, but without anything 
approaching to a shake. From the cylinder, at its mid length, project three arms by 
which it is held and levelled, each arm having through its extremity a cylindrical hole 
bored Q- inch diameter) parallel to the cylinder itself. This gun-metal cylinder, again, is 
supported by and held firmly to a strong and heavy plate of cast iron, which, having 
three bosses on its under surface, rests on one of the stone piers, part of the plate pro- 
jecting beyond the front of the stone towards the room. That part of the iron plate 
which rests immediately on the stone is a rectangle of 12 inches by 14 inches, and the 
projecting part may be described as something like an equilateral triangle of 8 inches 
