"Soumi;. ?uneTi«69?'] Coustruction of Objed-glasses. 343 
II. — On the Coiistruction of Object-glasses for the Microscope. 
By F. H. Wenham. 
{Continued from' page 2d9, No. V.) 
On the Production of Flat Surfaces in Glass. 
The most important tools required for this work are three circular 
cast-iron laps, ahout six inches in diameter, having a screwed boss 
at the back, similar to the face-chuck of a lathe. These must be 
first turned flat on their faces, and then scraped to a true surface, 
either from a standard planometer, as practised by engineers, or 
else the three may be scraped together till no error can be detected 
by their interchange. It would, perhaps, be out of place to give 
the details of this operation, which is described in most elementary 
works on mechanism. These planes, as left by the scraper, are not 
sufficiently smooth for the purpose required ; they must, therefore, 
be ground together. One of the plates is screwed down on a stud 
fixed in the bench or vice, and a wooden knob is fitted into the 
other to serve as a handle ; they are then rubbed together with fine 
emery and water, frequently interchanging the plates. It is a 
very difficult matter to bring these plates to an exact plane by 
grinding alone, and to keep them so during their continued em- 
ployment. The test of their truth is, that after they are all wiped 
clean and dry and rubbed together, the three should present a 
mottled appearance, uniformly covering the whole of their surfaces. 
One cause of error is a natural tendency of the grinding-powder to 
collect unequally between them. This may be somewhat corrected 
by frequently wiping it away from the places known to be hollow ; 
and the grinding 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, &c., on the laps, with very 
little injury to their plane figure, or even for the purpose of cor- 
recting it. In consideration of the extreme accuracy required in 
the prisms for spectroscope and other purposes, no pains should be 
spared in maintaining the perfection of these laps. 
If a number of discs of glass intended for smaU lenses are 
required to be ground and polished to a flat plane, they must be 
cemented to a block " — this is frequently merely a piece of wood 
turned with a convenient knob at the back for handhng ; others use 
a metal plate. Wood is handy for its lightness, but is liable to 
warp during the polishing operation, and so shift the discs; 
