MICROSCOPE. 
required for making the slightest adjustment. 
The axis of the body is never changed in the 
least degree,—which we have rarely found in 
rack adjustments, owing to the pressure of the 
pinion against one side of the bar; and thus an 
object is always kept in the centre of the field, 
whatever change made be made in its focal 
distance by an alteration of the magnifiers. By 
alittle practice, the power of making adjust- 
ments of extreme minuteness may be obtained, 
if the bar have been originally filed so true 
that it works with perfect smoothness in every 
ai An additional adjustment, which may 
made of any required fineness, is provided 
for, however, by making the square tube or 
socket itself moveable, and connecting it with 
a micrometer screw worked by a large milled- 
head at the bottom of the stem. By turning 
this screw, the socket, and the bar which it 
carries, are raised or depressed in any minute 
degree; and the socket being made to work 
through stuffing- boxes closely packed with 
cork, all twisting on its axis by the action of 
the screw is avoided. On the fineness of the 
serew the delicacy of the adjustment will of 
course depend. That which we have employed 
has forty turns to the inch; and the milled- 
head being 1% inch in diameter, or about 44 
inches in circumference, a movement of one- 
fiftieth of its periphery, or something less than 
#th of an inch, will affect the arm to the amount 
of s)oth of an inch. We should recommend, 
however, a screw rather finer than this. By 
dividing a circle on the milled head, and affix- 
ing an index-point to the bottom of the stem, 
the amount of motion given may be known to 
a great nicety, and thus the thickness of a mi- 
nute object laid upon any surface may be mea- 
sured. If its upper side be brought exactly 
into focus, and it be then removed, and the 
Surface on which it has lain be brought into 
focus by the micrometer screw alone, the num- 
ber of divisions over which the index has passed 
will of course indicate the thickness. This me- 
thod, which was proposed by Mr. Valentine,* 
answers very well Er such objects as the vessels 
of plants, scales of insects, &c. which can be 
completely isolated, when a sufficiently high 
power is employed, so that distinctness can 
only be obtained at one point. We consider it 
a great advantage of this kind of adjustment, 
that it is effected at a considerable distance 
from the stage, and that the hand is therefore 
in no danger of deranging the position of ob- 
jects or apparatus connected with it. 
The arm which carries the body is attached 
to the top of the bar by a screw-pin passing 
through the former, by which it is enabled to 
traverse from side to side. This movement 
will often be found extremely useful, both in 
the examination of different parts of objects 
which it is desirable not to move, and in chan- 
ging magnifiers, &c. on the body; the want of 
it we consider one of the chief inconveniences 
in Mr. Ross’s form of construction. The arm 
is jths of an inch thick, and broad enough at 
the part farthest from the centre to receive the 
* Trans. of Soc. of Arts. 
351 
body, which is not made tapering in the usual 
manner, but is attached by a screw of 14 inch 
diameter, with a large shoulder above it. By 
this mode of attachment, and by the massive- 
ness of the arm itself, all vibration from the top 
of the bar is prevented ; at least we have not 
been inconvenienced by it. When the bar is 
pushed into its socket, so that the arm ap- 
proaches the stage, (as when high powers are 
used,) no oscillation can arise from its vibra- 
tions; and the socket itself works through an 
aperture in the stage-plate, to which it is so 
closely fitted that no oscillation can arise in 
that point; so that, both in theory and prac- 
tice, we find the form here proposed unobjec- 
tionable on this score. It is true that, when 
the bar is drawn out to its full extent, oscilla- 
tion may arise; but this is never the case, ex- 
cept when low powers are being employed, and 
then we altogether fail to perceive it. In the 
construction of the body there is nothing worthy 
of peculiar remark; and, as we have already de- 
scribed the various modes of magnifying the 
object, we shall therefore now pass on to con- 
sider the best means of illuminating it. 
The perfect illumination of the object is a 
matter of the utmost importance, especially 
when high magnifying powers are being em- 
ployed. There are many difficult objects which 
require to be viewed under a great variety of 
aspects, in order that their true characters may 
be determined ; and there are not a few whose 
structure cannot be understood at all, even with 
the most perfect arrangement of the optical 
portion of the microscope, unless similar atten- 
tion be bestowed on the concentration of the 
light by which they are viewed. We shall, 
therefore, bestow on this subject more attention 
than it has ordinarily received. 
For transparent objects of large size, which 
are being viewed with low powers, such as sec- 
tions of wood, wings of insects, &c. we find a 
concave mirror by far the most simple and, at 
the same time, effective means of illumination ; 
and the optical errors to which it gives rise are 
not such as to interpose any practical difficulty 
in its use. It should be, for such a microscope 
as we have described, of greater size than that 
ordinarily employed ; three inches may be re- 
garded as a good diameter. It should be set, 
by an universal joint, upon a piece of tube 
fitted to slide stiffly up and down the stem 
which descends from the stage. In this man- 
ner its distance from the object may be readily 
varied ; and the degree of concentration of light 
effected by it will thus be easily adapted to the 
character of the object to be viewed. Thus, if 
it be pushed near the stage, the pencil of con- 
vergent rays will not be nearly brought to a 
focus, and a large surface will be illuminated 
with a moderate light. But if it be drawn 
nearer the opposite end of its range, the rays 
will be more concentrated, so that a smaller 
surface will be illuminated, but with much 
greater brightness. By the former adaptation 
we are enabled to illuminate, with great equa- 
lity, and by means of the ordinary lamp-flame, 
an area of three-eighths of an inch in diameter, 
with sufficient intensity to produce a very bright 
