Reflecting condenser for observing living bacteria etc. under dark ground illumination

UC-NRLF

B 3 IDA 2S7

mi

[reflecting condenser
k)r observing
living eacteria etc.
under dark ground

•T.LimiNATION. E.LEITZ

■

E. beitz, optical works, Uletzlar.

New-YorR

30, East 18th Str.

Berlin NW.

Luisenstrasse 45

Branches :

Chicago

360 Ogden Avenue

Frankfort o. M.

Neue Mainzerstr. 24

London W.

9—15 Oxford Str.

St. Petersburg

Woskressenski 1 1

Reflecting Condenser for Observing Living Bac-
teria etc. under Dark Ground Illumination.

Reflecting Condenser A.

This Reflecting Condenser, which was originated by Mr. W.
v. Ignatovvsky, a member of the scientific staff of the firm, was
fully described in the Zeitschr. f. wiss. Mikr., Vol. 25, 1908, No. 1.
It serves for observation under dark-ground illumination, and
more especially for bringing into view living and unstained bacteria.

This mode of illumination derives its success from the contrast
produced between the intensely illuminated bacteria and their dark
surroundings. Two reflecting surfaces, one internal, the other
external, as shown in the figure, are so shaped as to almost
completely unite the rays in a point P, and, by reducing the
astigmatism to its lowest limits, gives rise to an intense illumina-

piOLOGY
UBRARY

tion of the bacteria. Also, the apertures of the extreme rays a 1
and /j'PYie within the limits 1.1 and 1.45, from which it will be
seen that a considerable amount of light is collected at 'P. When
dry lenses are used all the rays which enter from below and
converge towards P go to illuminate the bacteria, as shown by lines
and dots, and are totally reflected at the surface of the cover-glass.
The light diffused by the bacteria, as represented by dotted lines,
enters the objective and thus produces an image of the bacteria,
which under these circumstances behave as self-luminous bodies.
Since the rays are united at P by reflection instead of by refraction,
there is no chromatic dispersion, and the annular illumination of
the bacteria obviates diffraction.

The light-gathering power of this reflecting condenser is such
that with an arc lamp fed by a continuous current of 4 amp. the
intensity of the illumination suffices for the photography of living
bacteria.

The optical portion of the dark -ground illuminator is contained
in a mount provided with a centreing arrangement and slips from
below into the sleeve which usually carries the Abbe condenser.

Since the point P should lie within the preparation it is neces-
sary to use slides of uniform thickness, not exceeding 1.0 mm.
The requisite correction is effected by raising or lowering the dark-
ground illuminator by means of the movement forming part of the
illuminating apparatus. It should however be noted that the space
below the object slide 0 should always be filled with oil.

The best source of light for the dark-ground illumination is
furnished by a small arc-lamp, but where this is not available a Nernst
lamp or incandescent gas lamp may be used, although, naturally,
the illumination will not be so bright.

The object may, as stated above, be viewed with dry lenses,
the illuminating rays being totally reflected from the upper surface
of the cover-glass, whereas only those rays which are dispersed by
the bacteria are allowed to enter the objective. High powers of the
dry series are better adapted for this purpose than lower powers,
partly owing to their higher apertures and their consequent greater
light gathering power, and partly owing to their greater magnifying
power. Owing to the influence which the thickness of the cover-
glass exercises upon the performance of high power dry lenses, it
is essential to employ cover-glasses of the proper thickness, viz.

0.17 mm, this being the thickness with respect to which the objec-
tives have been corrected.

Independence of the cover-glass thickness may be secured by
the use of objectives fitted with correction collars. The apochro-
matic objective of 4 mm focus and the achromatic lens No. S, which
are both supplied with a correction collar, are to be specially recom-
mended for this purpose. It may, in fact, be said generally that
apochromatic lenses are much to be preferred in dark-ground illu-
mination, since this mode of observation is extremely sensitive to
differences of colour.

The method is also available for observation with the aid of
immersion lenses, which offer a two-fold advantage. In the first
place, immersion lenses are within wide limits independent of varia-
tion in the thickness of the cover-glass, and in the second place,
the image of the object is brighter owing to the absence of reflections
at the surface of the cover-glass and of th"e-*front lens of the ob-
jective. When used in this way the immersion lens should be
stopped down sufficiently so that only the diffused rays enter the
eye, while the direct rays do not. Despite this limitation, the aper-
ture can be made greater than that of a dry lens, which again is
an advantage.

In the case of the Via" and lj\>"a lenses the necessary stopp-
ing is obtained by simply screwing a funnel into the objective.

As in the case of the dry lenses, the apochromatic lenses,
e. g. the apochromatic 2 mm or the fluorite 1n"a oil-immersion
lens, yield the finest and brightest images. In the case of the
2 mm apochromatic lens the reduction of the aperture has to be
applied with much greater care and cannot be effected by the ob-
server himself. The necessary modification does not, however,
render the objective useless for other purposes, for the stopping
devices may be removed by us.

Having screwed the objective to the microscope tube, slip the
dark-ground illuminator into the condenser sleeve, place the pre-
paration on the stage, and having placed a drop of oil on the top
of the illuminator, raise the illuminator until it touches the slide.

The light should now by means of a bull's eye lens be
directed upon the plane mirror. In the case of the 4 amp. arc lamp
listed below the illuminating lens is attached to the lamp casing
and the lamp is placed at such a distance from the microscope that
the observer may conveniently reach the carbon regulator whilst

312030

looking through the eyepiece. When a Nernst lamp or incandescent
gas light is used it is necessary to employ a bull's eye lens on
a stand, the distance between the source of light and the lens
being about 17 cm, that from the lens to the microscope mirror
about 40 cm. The pencil of light formed by the lens should
after reflection at the mirror give an image of the source of light
on the lower face of the dark-ground illuminator. The pencil of
light does not fill the entire mirror but rather its upper portion, it
being essential that the rays should be parallel to the axis of the
microscope and enter through the centre of the illuminator. Before
focussing the objective upon the object view the latter direct. A
bright spot will be seen upon the object, which, by raising or
lowering the illuminator, should be made as small as possible and,
by adjusting the position of the mirror, directed into the middle of
the upper surface of the illuminator, when its maximum brightness
will be attained. The object should now be focussed and viewed
with a very low eyepiece, say No. 0. It will generally be found
that the brightest point is not situated in the centre of the field.
By centreing the sleeve, the point P should be brought into the
middle of the field. The low power eyepiece may now be replaced
by one of higher power, the compensating eyepiece No. 18 being
particularly suitable for this purpose, and, if necessary, the correct-
ion collar of the objective should be adjusted and the dark-ground
illuminator raised or lowered with the aid of the condenser move-
ment, until the bacteria are as bright and the field as dark as
possible.

Observation with dark-ground illumination renders it imperative
that the object-slide and cover-glass should be cleaned with the
utmost care, otherwise the presence of particles of dust may inter-
fere with the observation. The preparation itself should be very
thin, i. e. present as little substance as possible, to prevent
the particles lying outside the plane of observation from giving rise
to disturbing reflections. For similar reasons it is essential to ensure
the absence of air bubbles.

The lower movable iris diaphragm which forms part of the
Abbe illuminating apparatus should be thrown out of action during
observation with the dark ground illuminator.

In addition to the Reflecting Condenser just described a simple
form of it is made consisting of a plate in which the condenser

is mounted and which may be placed upon the ordinary stage of
the microscope, as shown in the subjoined figure.

In this form the Reflecting Condenser need not be specially
adapted, but is ready for use with any microscope. The Condenser
Plate is merely placed upon the stage and held in position by the
ordinary clips. Large stands have as a rule centreing stages but
in their absence it is not a very difficult matter to centre the con-
denser plate by hand. To facilitate this operation the upper surface
of the condenser has ruled upon it two small concentric circles,
which should be brought into the centre of the field of a low power
lens. The axis of the condenser is thereby made to approximately
coincide with the axis of the microscope. The final adjustment
should, after the manner described above, be effected under a high
power. The adjustment of the Reflecting Condenser along the axis,
which is of the utmost importance, is accomplished with the aid
of the lever shown in the illustration of the Reflecting Condenser
B. The latter should be so placed upon the microscope stage that
the lever is towards the observer.

Reflecting Condenser B.

Prices.

Reflecting Condenser A with Centreing Device .... 16 —
Sleeve for the above — .40

(This is necessary in the case of those stands only which
are fitted with a swing-out condenser, and accordingly have
no sleeve. After swinging the condenser out and opening the
upper iris diaphragm to its full extent the sleeve is slipped
in from below in the place of the condenser and the dark-
ground illuminator introduced into the sleeve.)

/Reflecting Condenser B 16.—

>/felectncal Arc Lamp, with hand feed for a current of
4 amps, and illuminating lens mounted on stand for
connection to any existing house supply .... 16. —

^Resistance for 110 Volts 6.—

Nernst Lamp on Stand #"— SO-

Incandescent Gas Lamp on Stand 6. —

Illuminating Lens on Stand, (Bull's Eye Condenser) . . 8. —

to be used with Nernst Lamp or incandescent gas light

Apochromatic Objective 4 mm with correction collar. . 48. —

Achromatic Objective No. 8 with correction collar . . 20. —

Eyepiece 0 2. —

Compensating Eyepiece No. 12 10.—

Compensating Eyepiece No. 18 8. —

/Funnel, screwing into the 712" or l/iz'a oil immersion

lenses — .40

Electrical arc lamp with hand feed for a current of 4 amps, and
illuminating lens mounted on stand.

^.

.^

Ferd. Schnitzler Wwe. u. Kinder

Wetzlar

^

Directions for use of Dark fie! aratus.

1. Use very thin films, with few bubbles.

2. Have alllenses very clean.

3. Attac . :us.

4. Remove all other condensers.

5. Funnel stop in oil immersion obj .

6. Center condenser with #3 obj.

7. Drop of oil on condenser ana put sliae on

oil. No^ oil on slide.

8. Examine with low power. Center light by

mirro r.
Focus condenser by lever.

9. Oil on 1/12 - swin:.: in obj.
Proceed

Feep lens on lamp iri place.

row light on upp -*r part of mirror.
Use train slide.
Do not use scratched slides.
5' pal slide wi t h vas el ine
Oil s: o uld be cl ear .

UNIVERSITY OF CALIFORNIA LIBRARY,
BERKELEY

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

Books not returned on ti?e are subject .u , .Jne of
50c per volume after the third day ove , .q

Z^lZltTne^Jt^^olis made before
expiration of loan period.

„UG 30 193

MJG

JWrfi^M**

75m-7,'30

312030

•'OLOGY
LIBRARY

UNIVERSITY OF CALIFORNIA LIBRARY