ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
473 
cumference of this orifice has underneath a slight projection bearing 
a glass plate, the whole forming a chamber of slight depth (about 
0*75 mm.). The function of the chamber is to regulate the flow of the 
homogeneous-immersion medium, so as to avoid injury to the object by 
Fig. 80. 
alteration of the position of the cover-glass. A screw and drumhead 
divided into degrees works in an axially perforated cylinder, and effects 
the rotation of the chamber. 
A list of advantages claimed for the apparatus closes the description. 
Two Old Microscopes exhibited by the President at the last 
Meeting. — The first (fig. 81), which was made by Benjamin Martin, is 
an attempt to improve Cuff’s “ new constructed double Microscope ” 
(1744), with what success we shall presently see. The instrument, 
which is mounted on a folding tripod in place of the square box foot, 
differs essentially from Cuff’s, as it is a stage focuser. Cuff’s Microscope 
had a sliding coarse adjustment, the limb which carries the body sliding 
on a square pillar fixed to the box foot. A jamb screw was attached 
for the purpose of fixing the limb at a mark engraved on the square 
pillar, corresponding to the number on the objective on the nose-piece. 
Alterations in focus were then made by the fine-adjustment screw, which 
caused the body either to approach or recede from the stage. 
In Martin’s the body remains fixed while the stage is moved up or 
down by rack and pinion, until the stage is brought to a mark engraved 
on the fixed pillar, corresponding to a number on the objective. 
Parallel to the fixed pillar is a square steel bar, to which the stage may 
be clamped by a jamb screw. When the fine-adjustment screw is turned, 
it moves this square steel bar up or down, carrying with it the stage, 
and in doing this, let it be noted, it turns the coarse-adjustment pinion ! 
This is, of course, a much inferior method to that of Cuff’s, both on 
account of the stage focusing, and also with regard to the manner it is 
effected, which causes a superabundance of friction. The loss of time 
on the fine-adjustment screw amounts to no less than one and a half 
revolutions ! 
There are six objectives, with a single lieberkuhn common to them 
all ; the lieberkuhn slides on a long nose-piece, after the manner of 
Cuff’s, marks being engraved to indicate its proper position with any 
given objective. 
There are also six magnifiers for use as simple Microscopes ; three 
of these are fitted with proper lieberkuhns. There is an ordinary spring 
slide-holder, as well a supplementary stage with transverse motion 
in arc, an arrangement peculiar to Martin’s Microscopes. The limb 
