hey will
depend on the detailed structure of the lathe. Use the slide rest as
before, i.e. let it grasp an ordinary turning tool lightly, the
pedestal being fixed, but the rest free to slide up or down the lathe
bed. Push the back rest up till the butt of the turning tool (ground
to a rounded point) rests against the concave grinding tool. If the
diameter of the convex tool required be very small compared with the
radius of curvature of the surface (the most usual case), it is only
necessary to feed the cutting tool across to "copy" the concave
surface sufficiently nearly.

Fig. 49.

There seems no reason, however, why these methods should not be
applied at once to the glass disc by means of a diamond point, and the
rough grinding thus entirely avoided. I am informed that this has
been done by Sir Henry Bessemer, but that the method was found to
present no great advantage in practice. A reader with a taste for
mechanical experimenting might try radius bar tools with small
carborundum wheels rapidly driven instead of a diamond.

Enough has now been said to enable any one to prepare rough convex or
concave grinding tools of iron or lead, and of the same diameter as
the glass to be ground.

The general effect of the process of roughing the rotating lens
surface is to alter the radius of curvature of both tool and glass;
hence it is necessary to have for each grinding tool another to fit
it, and enable it to be kept (by working the two together) at a
constant figure. After a little practice it will be found possible to
bring the glass exactly up to the required curvature as tested by
template or spherometer. The art of the process consists in altering
the shape of the grinding tool so as to take off the glass where
required, as described in Sec. 53, and from this point of view lead has
some advantages; (opinions vary as to the relative advantages of lead
and iron tools for this purpose, however). The subsidiary grinding
tool is not actually needed for this preliminary operation, but it has
to be made some time with a view to further procedure, and
occasionally is of service here.

Sec. 57. 'The glass disc must be ground approximately to the proper
curvature on each side before any fine grinding is commenced. It is
precisely for this purpose that the previous turning of the disc is
recommended, for it is easy to unmount and recentre a round object,
but not so easy if the object have an indefinite shape. U