I should
think that the whole lens surface would require to be repolished or
perhaps reground in this case.

F. To decide in which surface faults exist is not easy. By placing a
film of oil between the two surfaces nearly in contact these may be
easily examined. Thus a mixture of nut and almond oil of the right
proportion, to be found by trial, for the temperature, will have the
same refractive index as the crown glass, and will consequently reduce
any errors of figure in the interior crown surface, if properly
applied between the surfaces. Similarly the interior of the flint
surface may have its imperfections greatly reduced in effect by using
almond oil alone, or mixed with bisulphide of carbon. The outer
surfaces, I presume, must be examined by warming or cooling over
suitable areas or zones.

The defects being detected, a matter requiring a great deal of skill
and experience according to Sir H. Grubb, the next step is to remedy
them; and the remedial measures as applied to the glass constitute the
process of figuring. There are two ways of correcting local defects,
one by means of small paper or pitch covered tools, which according to
Sir H. Grubb is dangerous, and according to the experience of Mr.
Cook, and I think of many French opticians, safe and advantageous.

Pitch polishing tools are generally used for figuring. They are made
by covering a slate backing with squares of pitch. The backing is
floated with pitch say one-eighth of an inch thick. This is then
scored into squares by a hot iron rod. The tool, while slightly warm,
is laid upon the lens surface, previously slightly smeared with dilute
glycerine, until the pitch takes the figure of the glass. The
polishing material is rouge and water. Small tools are applied
locally, and probably can only be so applied with advantage for grave
defects.

The other method is longer and probably safer. It consists in
furnishing the polishing tool with squares of pitch as before. These
being slightly warm, the lens is placed upon them so that they will
flow to the exact figure also as before. I presume that the lens is
to be slightly smeared with glycerine, or some equivalent, to keep the
pitch from sticking. The squares are most thickly distributed where
the abrasion is most required, i.e. less pitch is melted out by the
iron rod. This may be supplemented by taking advantage of differences
of hardness of pitch, making some squares out of harder,