hing, etc,
have to be gone through and in the manner described, and when all this
is accomplished the final process of correcting to test commences.
This process is called figuring.

Sec. 67. Of the actual operation of this process I have no personal
knowledge, and the following brief notes are drawn from the article by
Sir H. Grubb, from my assistant's (Mr. Cook) experience, and from a
small work On the Adjustment and Testing of Telescopic Objectives, by
T. Cook and Sons, Buckingham Works, York (printed by Ben Johnson and
Co, Micklegate, York). This work has excellent photographs of the
interference rings of star images corresponding to various defects.
It must be understood that the following is a mere sketch. The art
will probably hardly ever be required in laboratory practice, and
those who wish to construct large telescopes should not be above
looking up the references.

The process is naturally divided for treatment into two parts.

(1) The detection of errors, and the cause of these errors.

(2) The application of a remedy.

(1) A lens, being mounted with its final adjustments, is turned on to
a star, which must not be too bright, and should be fairly overhead.
The following appearances may be noted:-

A. In focus, the star appears as a small disc with one or two rings
round it; inside and outside of the focus the rings increase in
number, are round, concentric with the disc, and the bright and dark
rings are apparently equally wide. The appearance inside the focus
exactly resembles that outside when allowance is made for chromatic
effects. Conclusion: objective good, and correctly mounted.

B. The rings round the star in focus are not circular, nor is the
star at the centre of the system. In bad cases the fringes are seen at
one side only. Effects exaggerated outside and inside the focus.
Conclusion: the lens is astigmatic, or the objective is not adjusted
to be co-axial with the eyepiece.

C. When in focus the central disc is surrounded by an intermittent
diffraction pattern, i.e. for instance the system of rings may appear
along, and near, three or more radii. If these shift when the points
of support of the lens are shifted, flexure may be suspected.

D. On observing inside and outside the focus, the rings are not
equally bright and dark. This may be due to uncorrected spherical
aberration, particularly to a fault known as "zonal aberration," where
different zones of the lens have