Microscope with new Focussing Mechanism „ 1 1 1 
engages the body-tube, it is evident that tbe body-tube requires only 
one planed slide for both adjustments. In order to render the levers 
more compact, the coarse adjustment lever is made of two separate 
plates, between which the tine adjustment lever lies. A spring, not 
shown in the drawing, keeps the body-tube firmly pressed against the 
end of the lever C. 
The milled heads controlling both adjustments lie close together 
at the back of the instrument, in a convenient position for the hand. 
To give clearance for the milled head of the substage coarse adjust- 
ment, the limb is divided into two parts from the stage upwards for a 
distance of about two inches. This arrangement is found also to give 
the stage greater rigidity relatively to the limb. 
It is a noteworthy feature of the instrument that all the milled 
heads controlling the body-tube and substage have their bearings in 
the main casting ; consequently the pressure of the hand on the heads 
cannot affect the adjustment of focus. This offers an obvious advan- 
tage over the ordinary Jackson model with rack-and-pinion ; for in the 
latter the milled heads of the coarse adjustment are almost invariably 
carried by the fine adjustment slide. 
The description of the focussing arrangements of the body-tube 
applies throughout equally well to those of the substage. The motions 
are precisely similar, the levers being in fact interchangeable. To 
ensure rigidity, the limb, stage, and tail-piece are made in one casting. 
In the first experimental instrument made, the fulcrum of the fine 
adjustment lever was raised directly by a screw, without the interpo- 
sition of a second lever. This arrangement, though perhaps simpler, 
was abandoned in the instrument shown before the Society, since it 
was found not to give a sufficiently fast motion to the coarse adjust- 
ment. In the instrument shown, five revolutions of the coarse adjust- 
ment head, or two hundred and eighty of the fine, move the body- 
tube through an inch. The range of motion of the coarse adjustment 
is an inch and a half, that of the fine an eighth of an inch. Since 
the motion is limited only by the compressibility of the spring used, 
and not by the levers themselves, a larger range could probably be 
obtained without difficulty, were it considered necessary. The levers, 
independently of the spring, would admit of a motion of two inches 
and a half. The range at present obtained is, however, found suffi- 
cient for all work with ordinary objectives. 
Other details of the instrument, such as the foot, the stage, and 
substage, are more or less of ordinary patterns, and call for no special 
comment. 
