


Tur NoRTHERN MuIcROScoPIsT. 


No. 18. JUNE. 1882. 


PREPARATION OF TRANSPARENT SECTIONS 
OF ROCKS AND MINERALS: s 
‘Byglim@s-SORBY sls. dD nme ae oes Occ: 
( Concluded. ) 
HEN you commence to grind down you can do it with your 
eyes shut and without any trouble; and you find it easy, if it 
touches at one part and not at another, to humour it until the 
specimen is ground down evenly and the zinc touches at all four 
corners. 
Then comes the finishing off, which is really the troublesome 
point. You take off the zinc and must use more care. There is not 
so much difficulty in keeping it even, and now you see the advan- 
tage of having the glass made square. Instead of having to keep 
constantly looking you can tell at once by the feel whether the 
rock is uniformly thick. You have to rub down on the stones that 
I have described, and ultimately finish off on the very fine Water- 
of-Ayr stone, and leave the section of the thickness desired, a 
matter of course depending on the circumstances of the case. In 
some instances the section ought to be not more than the z,55 of 
an inch in thickness, but in the case of other rocks you would not 
be able to learn what you wanted from a section of that thickness, 
you would have to leave it thicker. With very fine grained lime- 
stone or slate—a roofing slate, say—you must have the section 
exceedingly thin or you would learn nothing at all, because you 
require a power of 4oo linear to explore such rocks, .and unless 
the sections are exceedingly thin you would not be able to study 
the character of the individual constituents. There are no large 
fragments ; all the material is finely divided, and you must have 
the section thin or one constituent hides another. 
If you want to study a fine grained material make the section 
thin. On the contrary, if you want to study the optical characters 
of pa of the larger fragments of minerals that occur here and 
Ole 2s 

