66 PREPARATION OF SECTIONS OF ROCKS AND MINERALS. 
the delicate features are obliterated, and even the delicate 
inner structure of the coarser outlines is gone. 
Let me mention a practical illustration. I have been 
studying an obscure microscopic coralbyslicingit. Ithad 
benconsiderably studied, both in Canada and in Europe, 
by eminent scientists who had failed to find any tabule 
and who had so reported. In my earlier sections, where 
I fell into this common error of extreme thinness, I found 
none; subsequently, by leaving my sections at a thicker 
stage, I found the tabule standing out in absolute 
clearness. Some of the scientists who had been unable 
previously to discover them ; after seeing my own speci- 
mens, were able to barely distinguish them also in their 
thin specimens, by very careful scrutiny. 
The rule of practice is quite evident. No one stage of 
thinness will suffice for both the coarser, and the more 
delicate structures. Watch the slice carefully as it be- 
comes more and more translucent, studying especially 
the more delicate structures first. To develop these, 
stop at exactly the point where more thinning seems not 
to bring them out better, but to lessen their clearness of 
contrast: Lay this slice aside as one specimen ; prepare 
other slices with a greater degree of thinness, to develop 
better the coarser structures. The conditions of each 
kind of specimen can alone determine how many slices, 
of as many different grades of thinness, may be required 
to develop completely the structure of any one object. 
Another remark is here important. To some extent 
this precaution may often be sufficiently observed even 
where only a single slice is made; as for instance when 
the material is scarce, or where the object really cannot 
be duplicated. It may be done in this way: stop the 
general grinding and thinning of the slice on the plate 
glass, exactly the point where the more delicate tissues 
have their maximum of clearness. Then, by using a 
proper tool, thin out by delicate manipulation only the 
ike 
