58 
THE YOUNG SCIENTIST. 
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, con- 
sisting 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 pres- 
sure. If the smoothing has been properly 
done, as directed, a few minutes will suf- 
fice to give the requisite polish, which is 
seen to take place equally all over the 
glasses ; but if any scratches should de- 
velop 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 in- 
stead of numerous pieces. 
Fig. 7. 
Fig. 8. 
For minute prism work, where the size 
is required to be only just sufQcient to 
transmit or reflect the pencils from a 
microscope object-glass, and the surface 
has to be perfectly up to a sharp edge, 
somewhat different practice must be 
adopted; for however carefully the 
smoothing or polishing may be per- 
formed, a rounding of the extreme edge 
always occurs. To obviate this, the edges 
must be guarded, as in the following 
examples: — A (Fig- 7) is a prism to be 
worked to a very acute angle. A piece of 
glass (Fig. 7), large enough for the pur- 
pose, having one side polished, is ce- 
mented with Canada balsam to a parallel 
plate of glass (B) ; they are then ground 
off together to the required angle and 
polished ; the marginal error will be taken 
up by the lower end of the under plate. 
It would be impossible to make an acute 
wedge of this figure in any other way, and 
when separated it will be found to have a 
knife-edge perfect in the extreme. 
Another example may be described from 
my practice in making the first prisms for 
the binocular microscope. A (Fig. 8) is 
an end view of the intended prism ; this is 
supposed to have been a block of glass of 
larger size, with one polished surface ce- 
mented with Canada balsam on to the 
guard plate (B); the front and back re- 
flecting surfaces are then smoothed and 
polished; these are then covered with 
guard-plates, and the top emergent sur- 
face of the prism ground off and polished 
to the dotted line (C, C). It will thus be 
seen that every corner of the prism is pro- 
tected during the working, and is kept ab- 
solutely perfect to the edge. The prisms- 
were made sufficiently long to be cross-cut 
into three or four. The smoothing was. 
performed in accordance with the fore- 
going directions, but the polishing lap was 
required to be much smaller. The one I 
employed was only Ih inches in diameter. 
If a large lap is used, the polish is apt to 
commence on the margins of the glass; 
and if this is the case, a true reflecting 
figure will never be obtained. The polish 
should begin in the centre and spread to 
the outside. The proper angles for these 
prisms were set off by a graduated steel 
sector, and, as the measurements have 
to be taken from the back of the guard- 
plates, it is necessary that these should be 
exactly parallel ; if not so, they must be 
ground on the surface-laps till all the 
edges gauge alike. 
I may here remark that I ani merely re- 
cording what has been my own self-ac- 
quired practice, and which is perhaps 
neither the most expeditious nor easy. 
My best apology must be, that I have 
always secured perfectly accurate results 
by these methods, and when a few only 
are required, I must confess that I do not 
