^?o"unmi,SriS?] 0/ Ohjed-Qlasses for the Microscope. 345 
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 purpose, 
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 inch asunder, are now turned in the wax, and 
some cross scratches made radiating from the centre, from which a 
piece should be taken out. The polishing powder, consisting of 
a mixture of crocus and putty powder before described, should be 
mixed in a small gallipot with plenty of water, and applied to the 
lap with a feather. The lathe is now run at a pretty quick speed, 
and the block of glasses worked over it in every direction with 
considerable pressure. If the smoothing has been properly done 
as directed, a few minutes will suffice to give the requisite polish, 
which is seen to take place equally all over the glasses ; but if any 
scratches should develop themselves, it is better to repeat the 
smoothing than attempt to polish them out. This same method is 
employed if the glass were one continuous plane instead of numerous 
pieces. 
For minute prism work, where the size is required to be only 
just sufficient to transmit or reflect the pencils from a microscope 
object-glass, and the surface has to be perfect up to a sharp edge, a 
somewhat different practice must be adopted ; for however carefully 
the smoothing or pohshing may be performed, a rounding of the 
extreme edge always occurs. To obviate this the edges must be 
guarded, as in the following examples. A, Fig. 1, is a prism to 
Fig. 1. 
be worked to a very acute angle. A piece of glass large enough for 
the purpose, having one side poHshed, is cemented with Canada 
balsam to a parallel plate of glass, B ; they are then ground off 
