376 MICROSCOPY. 
fine copper or brass wire, suitably bent, is placed with one end on 
the slide in the centre of the drop and the other end lying firmly 
on the table. Crystallization commences immediately, and when 
it is sufficiently advanced the wire is removed, the remaining 
liquid poured off, and the crystals washed with a drop of distilled 
water (containing a trace of gum to fix the crystals on the glass) 
and mounted dry in a cell. The time required to obtain the best 
results will be learned by a few trials, and a variety of specimens 
may be obtained by varying the strength of the solution or the 
size of the wire. 
Artirictat Fossirs. — Metallic casts, preserving perfectly the 
microscopic structure of the wood, may be formed by the defla- 
gration of nitrate of silver on charcoal. The process, as described 
by Dr. Chandler, consists in directing a blow-pipe flame upon à 
piece of charcoal upon which a crystal of the nitrate is lying. 
When deflagration commences, crystals may be successively 
added, and the silver, as reduced, replaces the particles of char- 
coal until a perfect copy, in metallic silver, is produced. 
NOMENCLATURE or Oxsectives. — The following remarks arè 
suggested by the points discussed in Dr. Ward’s paper om yer 
subject in the March number of the Narcrauist. Dr. Ward well 
says of the prevailing practice. ‘To call two lenses of identical 
magnifying power respectively one-fourth and one-sixth - 
lenses, is just as indefensible as to call two houses of equal height, 
forty and fifty feet high respectively.” An apt illustration of this 
is afforded by Dr. J. J. Woodward’s paper in the April number, 
where he cites an instrument invoiced zy, which by actual ger 
ment at the ‘open point’ was only a ṣẹ? But other objective? 
by the same maker called ;4,, are known to be nearly << 
focus as 4 or less than 1, showing that no system is used in 
nomenclature. Other instruments from other makers have it 
similar results. With such discrepancies, and confusion, pres 
copists have nothing to depend on in ordering or companha = 
instruments. . +. the : 
The points from which the measurements are to be edy the 
question which is most obscure. Undoubtedly the best, - d 
true theoretic plan is to measure from the optical centre ei 
objective to the optical centre of the ocular, oF we woa 
focus. This is the method of one maker of objectives: ; 
