masters, it is beyond the scope of the present paper, and even now I can
only hope to call your attention to one phase of this important subject.
For a number of years I have been practically interested in the subject of
the production of plane and curved surfaces particularly for optical
purposes, _i.e._, in the production of such surfaces free if possible from
all traces of error, and it will be pleasant to me if I shall be able to
add to the interest of this association by giving you some of my own
practical experience; and may I trust that it will be an incentive to all
engaged in kindred work _to do that work well?_

[Illustration: FIG. 2.]

In the production of a perfectly plane surface, there are many
difficulties to contend with, and it will not be possible in the limits of
this paper to discuss the methods of eliminating errors when found; but I
must content myself with giving a description of various methods of
detecting existing errors in the surfaces that are being worked, whether,
for instance, it be an error of concavity, convexity, periodic or local
error.

[Illustration: FIG. 3]

A very excellent method was devised by the celebrated Rosse, which is
frequently used at the present time; and those eminent workers, the Clarks
of Cambridge, use a modification of the Rosse method which in their hands
is productive of the very highest results. The device is very simple,
consisting of a telescope (_a_, Fig. 1) in which aberrations have been
well corrected, so that the focal plane of the objective is as sharp as
possible. This telescope is first directed to a distant object, preferably
a celestial one, and focused for parallel rays. The surface, _b_, to be