344 
On the Construction 
t Monthly Microscopical 
Journal, June 1, 1869. 
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 common black sealing-wax. For a small series of 
discs, a block of about two inches in diameter will be found most 
manageable. The pieces of glass cemented on to 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 fine 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 direction, 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 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 become dry ; remove the block and wipe all the emery 
away about three-eighths of an inch from round the circumference 
of the lap ; breathe on it again ; continue the smoothing, 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 poHshing 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 do this, and the operation must be 
repeated again and again, till no scratch whatever can be discovered. 
It is quite evident that to obliterate a scratch by polishing, the 
whole surface must be worked away till the bottom 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 imbedded. For 
facing the lap, I employ bees- wax hardened with resin, and stir 
some finely-washed ochre into the melted mixture. The lap itself 
is simply a brass plate, about three inches in diameter, which screws 
on to the lathe mandril ; some of the above material is poured on to 
this, and spread into a layer of about one-sixteenth of an inch thick. 
