104. THE NORTHERN MICROSCOPIST. 

the same curvature, when you come to rub them down they are 
worn evenly until they are perfectly flat, and you have really then 
got two small flagstones with a comparatively plane surface. 
You require a number of different kinds of stones to work your 
specimens upon. I first used to apply sandstone with emery, but 
this was inconvenient and wore hollow; but I have found that a 
convenient method was to do the rough grinding on a plate of 
zinc about a foot square and hammered flat, rubbing the specimens. 
down on this zinc first with fine ground emery and then finishing 
them off with finer still. I had two zinc plates, one worn somewhat 
hollow and the other as flat as possible, doing the rough work on 
the first and finishing it off on the more level with finer emery ; 
but you cannot get a flat surface in that manner. ‘The specimens 
always wear away more at the corners, and that is, perhaps, one of 
the greatest difficulties in the preparation of these objects, because 
it entails so much labour, but I do not see how it can be avoided. 
Having got the specimens dressed up in that manner, the next 
point is to get the surface into a far better condition than it can be 
got by emery, and I used to employ two or three kinds of stone. 
It is not always easy to get pieces of a satisfactory character, but I 
used to employ a kind of stone called Congleton stone, and also 
Water-of-Ayr stone. ‘This Congleton stone does not scratch, and 
it does not polish, but keeps a good cutting edge, so that you can 
rub down tolerably quickly. The specimens are finished off 
ultimately on the Water-of-Ayr stone, and for that purpose I used 
to have two portions, six inches square, one of a soft grain and 
the other a hard grain for finishing. ‘These stones are rubbed 
flat on the little flags that I alluded to. Having got these Water- 
of-Ayr stones, you grind both of them first on one stone and then 
on the other, so as to thoroughly equalise any irregularity. ‘Then 
these two were rubbed together, and by this means you get such a 
perfect plane that you can obtain far more accurate results than 
you require, but when you can get absolutely perfect results it is 
as well to do so. I find that if a drop of water was put on one 
stone and the other put upon it, it would float about as if on water, 
the perfection of the plane being such and the capillary attraction 
so powerful that the stones did not touch one another, but moved 
about quite readily. 
The specimens of rock roughly ground on the zinc plates were 
ground down on these stones, and then finished off on the harder. 
When you come to reflect on the question you will see that to get 
a perfect section of some rocks it is absolutely requisite to use all 
this care. : 
As an illustration of the perfection to which this method may 
be brought I may say that very often Limestones contain sand, and — 
there is not the slightest difficulty whatever in preparing a section 
