22 H.Goadby on making wet Preparations of Animal Substances. 
itable without this precaution ; it is true that beveling can be 
done at any time, but it is not easy to grind out the deep irreg- 
ular holes caused by splitting the edge. In this way, the four 
sides of the slide are to be ground. 
To keep the pewter plate flat, grinding should invariably be 
conducted all over its surface; but as this is somewhat difficult 
with small pieces of glass on a large surface of metal, I devote 
one side of my plate for slides alone, and reserve the other for pur 
poses where the utmost flatness is necessary. Its flatness, how- — 
ever, should be frequently tested with a straight edge, and if ele- 
vations appear, they should be reduced by grinding them down. 
Optical glass grinders and other mechanics, who require a plane 
grinding surface, have three similar tools; when one of them be- 
comes untrue by use, it is ground with one of the others, until 
they present a like surface, neither of them being true. Tool 
0. 1 being now ground with tool No. 3, the inequalities left by 
No. 2 are obliterated, and a flat surface is the result. As it would 
be particularly inconvenient for me to carry with me three plates 
each weighing 14 lbs. for the sake of keeping one of them true, 
I resort to another, but equally efficient plan: I take a piece of 
plate glass of the same length as the pewter plate, the width not 
being very material, with plenty of emery and water I grind the 
metal all over its surface with the flat side of the glass until they 
present a corresponding surface; if the metal be not sufficiently 
flat, I turn the glass and grind the other side: by this process 
the flatness of the metal may be insured. 
o abbreviate the time of edging the slides, it is expedient 
to hold one in each hand and grind them simultaneously ; and 
although this may be somewhat difficult at first, a little practice will , 
give all the facility and tact necessary for thus grinding two — 
_ glasses in the time of one. 
It failed; and for some months contin 
ventor made some specially for my use at the 
ued to fail, until the in-— 
College of Sur- — 
‘’ 
4 
