348 
should be paid to the action of this rack; it 
should work through a triangular socket at the 
top of the pillar of at least an inch long, closely 
embracing the bar, so that it may be moved up 
and down without the least shake, or variation 
from its axis, or tendency to slip, when loaded 
with the full weight it has to carry; at the 
same time, it should be sufficiently sensitive to 
be moved by the slightest rotation of the pinion, 
which may be effected, not only by the milled 
head, but by a lever, as in Mr. Holland’s mi- 
croscope. Such a rack will afford the means 
of focal adjustment for doublets and triplets of 
high power, as well as for ordinary magnifiers. 
If it be considered desirable to vary the length 
of the pillar, and consequently the height of 
the stage, this may be effected by attaching the 
rack, stage, &c., to a piece of tube which shall 
slide within the one that is attached to the 
base ; if the top of the lower one is sprung, and 
surrounded by a clamping screw, the upper one 
may be firmly fixed at any elevation. This 
plan has been proposed by Dr. Goring for the 
pillar of his engiscope; but we consider it per- 
fectly inapplicable to a large instrument, of 
which the pillar should be solid, in order to 
avoid oscillation. The triangular bar carries at 
its top a double arm, attached to it with a 
screw and a small milled head, by which its 
movements may be rendered more or less free. 
This arm should be thick and well hammered, 
in order that, when it carries the compound 
body, it may not be subject to vibration. The 
object of the double bend in one side of it is to 
allow the nose of the body, which is screwed 
into the hole on that side, to project below it, so 
that the magnifiers may be attached on its un- 
der side ; by this means, the necessity for un- 
screwing the body every time that the magni- 
fier is to be changed (which is requisite where 
the magnifier is screwed into the arm and the 
body into it) is avoided, without any increase 
in the length of the rack, which would other- 
wise be necessary for low powers. The other 
side of the arm carries the magnifiers, when 
employed as single lenses ; the same set may 
be used as are fitted to the compound body, 
their interior being screwed for attachment to 
it, and their exterior being adapted to drop 
loosely into the hole of the second arm. For 
such a microscope, we should recommend a 
series of ordinary lenses from 24 or 2 inches 
focus to }th of an inch, and two or three dou- 
blets from jth to jth, with a triplet of sth. 
These may all be used as objectives with a 
meniscus eye-piece ; though, for the latter, Mr. 
Holland’s will, in most cases, be preferable. 
The stage of this microscope may be a simple 
plate, attached to the top of the pillar, as in 
Jig. 166, with a spring stage and other appurte- 
nances adapted to it. We are inclined to re- 
commend for such a microscope, however, for 
the sake of simplicity and facility of use, a 
stage in which the ordinary loam of fixity 
is in some degree departed from. The upper 
part of it consists of a thick and strong fork, 
rojecting about 2 inches and about 1} inch 
bivoud) of well-hammered brass, firmly attached 
MICROSCOPE. 
to the top of the pillar ; at about §thsofaninch — 
plate of similar dimensions, — 
with an aperture of about an inch. 
upon this lower plate by a spiral ‘pring, aa 7 
below this is a 
guided by vertical pins working 
through aper. 
tures in it, is a thin moveable plate, whic i i, 
constantly being pressed upwards by ing 
against the fork. This stage combines, in a 
degree which renders it an extremely conve- 
nient one, the advantages of the ordinary spring- 
stage, and the fork. The objects, 
ee in sliders, on slips of glass, or in aquati 
xes, are readily slid under the ; 
fork (which should be bevelled off on the under 
side so as to allow them to enter), and are held 
by the spring with sufficient firmness in any 
position, whilst ready movement is also 
mitted them. The fork may have a versal 
hole drilled in it on each side, for holding the 
stage forceps or other such instrument; and it 
may be made thick enough to allow of a la- 
teral hole for fixing the condenser, ou 
being too much weakened. The thickness 
of the fork will be of no kind of inconve- 
nience, as the nose of the microscope will have 
free play from side to side between its prongs. 
= the lower plate may be attached ground- 
glass, stops, condensers, polarising a paratus, 
or any other required fiednge. The dendiviakt 
tage of this stage is that it affords no firm sup- 
port to the object, which is in some instances 
of importance. This pp be obtained by sim- 
ply adapting a plate to the upper side of the 
fork, which may be received into grooves on its 
lower surface; and this plate may then be con- 
sidered as the stage. Whenever such an in- 
strument is being adapted to the purpose of 
dissection, all that is n is to raise up a 
support for the hands on each side of the stage, 
which may consist of books, or still better of 
blocks of wood cut to the form of the outside 
of Mr. Holland’s case; and the height of the 
stage being then adjustible by the sliding tube 
of the pillar, all the advantage of Mr. Holland’ 
microscope, except its self-containedness, may be 
secured. We have only further to mention 
the mirror, which should be at least two inches 
in diameter, and have one of its sides , 
and the other concave. Its stem should be 
attached to a short piece of sprung tube slidi 
over the pillar, at capable of Bo 
in any situation by a clamping screw. wm 
If a still more portable microscope be re- 
quired, combining considerable range of Lge 
with great exactness of adjustment, we ¢é 
strongly recommend an instrument constructed 
upon the plan of the large one subse 
be described, (fig. 167,) but upon about two- 
fifths of the scale. Its pillar may either be — 
screwed into the lid of the box con th 
apparatus, or mounted on a — similar to 
that which is used for portable telescopes,—the 
tripod being reversed, and the legs being folded 
round the stem, when it is packed in its bo: 
The delicate focal adjustment of which thi 
microscope is susceptible renders it more ad. 
vantageous than the one last described, when 
deep magnifying powers are being employed ; 
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