138 THE NORTHERN MICROSCOPIST. 

to be sure that you are cutting perpendicular to the axis. But in 
the case of fluor spar, when there is no section of any particular 
interest you need not be so particular. You grind down one 
surface and polish it, grinding it very fine, and then you deal with 
the other ; and the point that I am especially dwelling on now is 
the method by means of which you can ascertain that you have 
got the sides approximately parallel. When grinding it down look 
at a crossbar, say of a window, and turn the section about, looking 
partly through the crystal and partly through the glass. If it is at 
all wedge-shaped the crossbar will be as it were thrown down, and 
you can ascertain the direction in which it is not parallel. If on 
_ looking in this way, and having ascertained that you have done 
something wrong and have corrected the error, if you again look 
at the crossbar and, turning the crystal any direction you like, find 
that the crossbar looks the same and does not undergo any change, 
it proves that you have got your section cut sufficiently parallel for 
all ordinary purposes. 
In cutting sections of minerals for the same purpose you must 
ascertain that you are looking in the particular direction that you 
want. If, for example, you are preparing a section of calcite so 
as to look right along the axis, you must take very great care to 
prepare a perfect rhombohedron to begin with, and then when 
roughly finished you must examine it to see that you are looking 
in the line of the axis. 
You may often see certain things very well indeed through 
minerals that are imperfectly polished by putting them in benzole. 
Supposing you had a portion of calcite rubbed down roughly, and 
you wanted to ascertain if it were cutting it in a proper direction, 
you would get some glass and a thin cover, put some benzole 
between this and the section. The benzole having nearly the 
same index of refraction as the calcite makes the rough surface so. 
transparent that you can ascertain then whether the section is cut 
in the proper direction or not. 
This method was very useful when looking at different loose 
sands. By having them in benzole you can study the minute 
particulars of grains of sand, and ascertain the portion of the 
fluid cavities that are contained in them. 
I am afraid I have given a very imperfect account of this subject. 
I have endeavoured to point out some of the principal difficulties, 
but I daresay I have left out a great many. For, when you have 
been for years practising an art, you do a great many things 
instinctively and do not know that they are difficult; it is only 
when you are beginning that you find them difficult. Especially 
in mechanical work you get by degrees not to realise points that 
perhaps might be difficult to others. But I hope that I have 
pointed out the principal facts and methods which I have employed. 

