, which, however,
from its simplicity deserves a note. Two pulleys, A and B, of about
18 inches diameter by 4 inches on the face, are arranged to rotate
about vertical axes, and belted together. The shaft of one of these
pulleys is driven by a belt in any convenient manner. Each pulley is
provided on its upper surface with a crank of adjustable length
carrying a vertical crank-pin.

Each crank-pin passes through a 3"X 2" wooden rod, say 3' 6" long, and
these rods are pinned together at their farther extremities, and this
pin carries the grinding or polishing tool, or rather engages loosely
with the back of this tool which lies below the rod. It is clear that
if the pulleys are of commensurable diameters, and are rigidly
connected--say by belting which neither stretches nor slips--the
polishing tool will describe a closed curve. If, however, the belt is
arranged to slip slightly, or if the pulleys are of incommensurable
diameters, the curve traced out by the grinding tool will be very
complex, and in the case of the ratio of the diameters being
incommensurable, will always remain open; for polishing purposes the
consummation to be wished.

Mirror surfaces are ground spherical, the reduction to parabolic form
being attained in the process of polishing. A very interesting
account of the practice of dealing with very large lenses will be
found in Nature, May 1886, or the Journal of the Society of Arts, same
date (I presume), by Sir Howard Grubb. The author considers that the
final adjustment of surfaces by "figuring"--of which more anon--is
an art which cannot be learned by inspection, any more than a man
could learn to paint by watching an artist. This is, no doubt, the
case to some extent; still, a person wishing to learn how to figure a
lens could not do better than take Sir Howard at his word, and spend a
month at his works. Meanwhile the following remarks must suffice; it
is not likely that anybody to whom these notes will be of service
would embark on such large work as is contemplated by Sir Howard
Grubb.

Fig. 57.

Description of Polishing Machine. Power is applied through belting to
the speed cone A. By means of a bevel pinion rotation is communicated
to the wheel D, which is of solid metal and carries a T-slot, C. A
pedestal forming a crank-pin can be clamped so as to have any desired
radius of motion by the screw E. A train of wheels E F G H K
(ordinary cast lathe change wheels) communicate a