1901] MICROSCOPICAL JOURNAL. 331 
require no substage apparatus or mirror, and althoughthe 
completed stand is usually employed, the demand has _ be- 
come sufficiently marked for makers to produce special 
instruments in which a large mechanicalstage is provided, 
but no fittings’ beneath the stage. Among these are Rei- 
chert, of Vienna, C. Baker, of London, and Queen, of 
Philadelphia. Inallof these the vertical illuminator plays 
an important part; some observers preferring the pattern 
with a prism, others the cover-glass reflector, while many 
employ both, finding that each pattern is advantageous 
according to the structure examined. 
When the light and illuminator haveonce been adjusted, 
itis important that no movement of the body of the mi- 
croscope takes place, or the illuminating would have to be 
re-set. To obviate this, these special microscopes by 
Queen and Baker have rackwork focussing adjustment to 
the stage, together with levelling screws, so that any want 
of parallelism in the piece of metal may be corrected, and 
the face set at right angles to the plane of the objective. 
It will be seen from this that the subject hasreceived care- 
ful consideration, and that suitable means are available for 
accurate work. 
The metals themselves that are to be observed have to 
be prepared with great care; the processes are technical, 
and vary with the purpose of the examination. Generally 
a small sample block is taken, and cemented toa pieee of 
glass. It is then ground and polished on aseries of emery 
papers, and finally with a very fine polishing material such — 
as rogue, until no scratches can be detected. 
Although the harder portions can be in many instances 
seen at this stage, chemical means have to be resorted to, 
to differentiate the structure, the action being unequal on 
the different constituents, and it is by the treatment with 
suchchemicals—nitric acid and liquorice juice being among 
the most important—that the chemical composition is de- 
tected. Many of the constituents have received distinct- 
ive names, among them being :— 
Ferrite, which is the name given to pure iron. 
Cementite, representing the iron carbide in steel. 
