THE YOUNG 
by frequently wiping it away from the 
places known to be hollow ; and the grind- 
ing together should be performed with as 
little powder as possible at a time, and the 
strokes so managed as to abrade the high 
parts only. Practical experience is the 
best guide for this ; and a clever workman 
will soon learn in what way and direction 
to work his blocks of glass, etc., on the 
laps, with very little injury to their plane 
figure, or even for the purpose of correct- 
ing it. In consideration of the extreme 
accuracy required in the prisms for spec- 
troscopes and other purposes, no pains 
should be spared in maintaining the per- 
fection of these laps. 
If a number of discs of glass intended 
for small lenses are required to be ground 
and polished to a flat plane, they must 
be cemented to a "block;" this is fre- 
quently merely a piece of wood turned 
with a convenient knob at the back for 
handling ; others use a metal plate. Wood 
is handy for its lightness, but it is liable 
to warp during the polishing operation, 
and so shift the discs ; to obviate this, I 
screw a flat piece of slate to the face of the 
wooden block, with a few common wood 
screws. 
The cement used for the glasses is either 
pitch hardened with some shellac, or com- 
mon black sealing-wax. For a small series 
of discs, a block of about 2 inches in di- 
ameter will be found most manageable. 
The pieces of glass cemented on this are 
arranged symmetrically, leaving as little 
interval between them as possible. They 
are now roughed down on the zinc plate 
till they are all brought to one level ; they 
are then washed with a nail-brush and 
well rinsed, and flne-ground on one of 
the laps, and next smoothed on a circular 
piece of cast-iron, but little exceeding the 
diameter of the block of discs. This 
smaller lap must be carefully ground to a 
true plane on the larger ones. A little of 
the finest washed emery and water is 
spread over this lap with a feather, and 
the glasses worked upon it in every di- 
rection, holding the lap in one hand and 
the block in the other, and occasionally 
turning both; this is continued till the 
emery begins to get dry, the glasses are 
then washed and wiped dry, and the 
SCIENTIST. 57 
smoothing proceeded with ; but no more 
water must be applied to the lap. This is 
now moistened by simply breathing on it. 
In a few minutes the lap will again be- 
come dry ; remove the block, and wipe all 
the emery away about iths of an inch 
from round the circumference of the lap ; 
breathe on it again ; continue the smooth- 
ing, and also wipe the emery away from 
the outside till, finally, scarcely any is 
left, and the glass is nearly finished on 
the metal itself. If this operation is 
properly conducted, the glass will have a 
transparent surface free from scratches 
and greys, and so near a polish that a few 
minutes only on the polishing lap will be 
required. But one rule must be strictly 
adhered to, viz., never to polish a glass 
surface with any scratches in it. It is 
worth while to spend any amount of time 
in smoothing rather than to do this, and 
the operation must be repeated again and 
again, tilt no scratch whatever can be dis- 
covered. It is quite evident that to ob- 
literate a scratch by polishing, the whole 
surface must be worked away till the bot- 
tom of it is reached. This makes the 
operation long and very tedious, and is 
almost certain to injure the perfectly flat 
plane which has been obtained by careful 
smoothing. 
It is a difficult and hazardous task to 
polish glass on hard metal, as the surface 
is very liable to tear up. Consequently, 
the usual system is to employ a soft and 
partly-yielding material, in which the 
particles of polishing powder may be im- 
bedded. For facing the lap, I employ 
beeswax hardened with resin, and stir 
some finely-washed ochre into the melted 
mixture. The lap itself is simply a brass 
plate, about 3 inches in diameter, which 
screws on to the lathe mandril ; some of 
the above material is poured on to this, 
and spread over a layer of about l-16th of 
an inch thick. When cold, it is turned 
off flat, and, to make it perfectly true, the 
whole face is scraped off at once with a 
hardened steel-cutting straight-edge. An 
old parallel cotter file will answer the pur- 
pose, ground from both sides like a blunt 
knife, and finally corrected on one of the 
cast-iron laps with emery. A series of 
shallow grooves, about an eighth of an 
