the surfaces by noting whether--and if so, to what degree--they
differ in appearance from point to point when held so that the light
falls on them obliquely.

It is necessary to exercise the greatest care in the washing between
the application of successive grades of emery, and this will be
facilitated if the edges of the glass squares were dressed on a
grindstone before they were mounted. An additional precaution which
may be of immense advantage is to allow the tool to dry between the
application of successive grades of emery (of course, after it has
been scrubbed), and then to brush it vigorously with a hat-brush. It
sometimes happens that particles of mud which have resisted the wet
scrubbing with the nail-brush may be removed by this method.

As my friend Mr. Cook informs me that his present practice differs
slightly from the above, I will depart from the rule I laid down, and
add a note on an alternative method.

Consider a single lens surface. This is roughed out as before by an
iron tool, a rough fellow tool being made at the same time. The
squares of glass are cemented to the roughing tool, and this is ground
to the spherometer by means of the counterpart tool. The glass-coated
tool is then applied to the lens surface and grinding with the first
grade of emery commenced. The curvature is checked by the
spherometer. Two auxiliary tools of, say, half the diameter of the
lens, are prepared from slate, or glass backed with iron, and applied
to grind down either the central part of the lens surface or tool
surface, according to the indications of the spherometer. Any changes
that may occur during grinding are corrected by these tools. The
spherometer is accepted as the sole guide in obtaining the proper
curvature. A slate backing is preferred for tools of any diameter
over, say, 2 inches.

Sec. 62. Polishing.

After the surface has been ground with the last grade of emery, and
commences to become translucent even when dry, the grinding may be
considered to be accomplished, and the next step is the polishing.
There are many ways of carrying out this process, and the relative
suitability of these methods depends on a good many, so to speak,
accidental circumstances. For instance, if the intention is to finish
the polishing at a sitting, the polishing tool may be faced with
squares of archangel--not mineral or coal-tar--pitch and brought to
shape simply by pressing while warm against the face of the