MICROSCOPE..- 
‘so that it may be free from vibration. An 
amount of vibration, which is imperceptible 
when low powers are used, is-sufficient to 
render the most perfect optical arrangements 
-for high magnifying powers next to useless, 
That the whole instrument should be secured 
-as much as possible from vibration, by being 
placed upon a steady table, and this on a 
Steady floor, is of the first importance for its 
advantageous employment. But, as an entire 
freedom from this cause of vibration can rarely 
be obtained in an ordinarily-constructed house, 
the next point to be aimed at is such an ar- 
rangement of the optical portion of the instru- 
ment in regard to the object, that any vibra- 
tion which takes place may affect both alike, 
‘in which case it will be scarcely perceptible. 
In many microscopes of ordinary construction 
the stage is comparatively motionless, whilst 
the body (containing all the optical portion) 
is subject to tremor; and in such instruments, 
when a high magnifying power is employed, 
the object is seen to oscillate so rapidly upon 
the slightest cause of vibration (such as a per- 
son walking across the room or a carriage rol- 
ling by in the street) that it is frequently almost 
indistinguishable. Various modes have been 
devised for obviating this inconvenience, the 
chief of which we shall hereafter notice. 
2. Capability of accurate adjustment to 
_ every variety of focal distance, without move- 
ment of the object. It is now a principle 
almost universally recognised in the construc- 
tion of good microscopes, that the stage on 
which the object is placed should be a fixture; 
and that the movement by which the focus is 
to he adjusted should be effected in the body 
or optical portion. Several reasons concur to 
establish this principle, which was, we believe, 
first insisted upon by Dr. Goring.* Among 
the most important we consider to be that, if 
the stage is made the moveable part, the ad- 
justment of the illuminating apparatus must 
be made afresh for every change of magnify- 
ing power, whilst, if the stage is a fixture, the 
illumination having been once well adapted, 
the object may be examined under a great 
variety of magnifying powers, without its being 
changed in any respect. Moreover, if the 
stage is the moveable part, it can never have 
that firmness given to it, which it ought for 
Many purposes to possess. It is almost im- 
possible to make a moveable stage free from 
some degree of spring; so that, when the hands 
bear upon it in adjusting the position of an 
object, it yields to an amount which, however 
trifling, becomes apparent with high powers 
by the alteration of the focus. We might add 
many more reasons, but these will here suffice. 
it having been determined, then, that focal 
adjustment should take place in the optical 
portion of the microscope, the next point for 
consideration is the mode of effecting it. This 
should be such as to allow free range from a 
Minute fraction of an inch to three or four 
* See Pritchard and Goring*s Microscopic Illus- 
trations, 
focus. 
345. 
inches, with equal power of obtaining a deli- 
cate adjustment at any part. It should also 
be so accurate that the axis of the instrument 
should not be in the least altered by movement 
in a vertical direction ; so that, if an object be 
brought into the centre of the field with a low 
power, and a higher power be then substituted, 
it should be found in the centre of its field, 
notwithstanding the great alteration in the 
In this way much time may often be 
saved by employing a low power as a finder 
for an object to be examined by a higher one; 
and, when an object is being viewed by a suc- 
cession of powers, no readjustment of its place» 
on the stage is required, such as would other- 
wise be necessary for each. The best modes 
of securing these ends, also, will be considered 
in their proper place. 
3. The power of placing the instrument in 
either a vertical or a horizontal position, or at 
any angle with the horizon, without deranging 
the adjustment of its parts to each other, and 
without placing the eye-piece in such a posi- 
tion as to be inconvenient to the observer. It 
is certainly a matter of surprise that opticians 
should have so long neglected the very simple 
means which are at present so commonly em- 
ployed, of giving an inclined position to mi- 
croscopes ; since it is now universally acknow- 
ledged that the vertical position is, of all that 
can be adopted, the very worst. We do not 
ourselves consider the horizontal position of 
the body and its appendages at all an advan- 
tageous one, although it has been adopted in 
some of M. Chevalier’s latest and best instru- 
ments. In the first place it requires that the 
whole microscope should be raised so much 
above the level of an ordinary table, as to 
bring the eye-piece to the height of the eye of 
the observer when sitting upright at his ease; 
if this be not done, a constrained and conse- 
quently disadvantageous position of the head 
is required on his part; and if it be, all the 
manipulations must be executed at an elevation 
very inconvenient and fatiguing to the arms. 
Moreover either the stage must be rendered 
vertical, in which case all the objects must be 
so secured (to prevent their slipping) as to 
render the necessary movement of them very 
difficult; or, the stage being horizontal, the 
direction of the rays must be changed, after 
they have passed through the object-glass, by 
a prism ora mirror placed at an angle of 45° 
in their course, as in M. Chevalier’s construc- 
tion, which we think a decided disadvantage, 
as introducing another source of imperfection 
and error. We believe that it will be gene- 
rally acknowledged, that an inclination of about 
45° to the horizon is the most convenient for 
unconstrained observation ; and the instrument 
should be so arranged, that, at such an incli- 
nation, the stage may be so far elevated above 
the table, that, when the hands are employed 
at it, the elbows may rest upon the table. In 
this manner a degree of support is attained, 
which gives such free play to the muscles of 
the hands, that movements of the greatest 
nicety may be executed by them; and the 
