Abbe’s Condenser.
The essential feature of this condenser is its short focus, which collects the light reflected by the mirror, so as to form a cone of rays of very large aperture, having its focus in the plane of the object.
Fig. 128.—The Iris Diaphragm, and carrier for Stops.
The full aperture of the illuminating cone should only be used when finely granular and deeply stained particles (protoplasm, bacteria, &c.) are being examined with objectives of large aperture. In all cases the cone must be suitably reduced, either by an iris, or other form of diaphragm (central illumination). By placing the diaphragm excentrically, by means of rack-work attached to the carrier, the central rays are excluded and a certain extra-axial portion of the illuminating pencil falls upon the object (oblique illumination). When the diaphragm is thus excentrically placed, this oblique pencil can be directed from all sides by rotating the carrier round the optic axis. The central stop diaphragm shuts off all the axial and transmits only the marginal rays, thus producing dark-ground illumination. The iris diaphragm ([Fig. 128]) is so shaped that the edge of its smallest opening closely approximates the object-slide on the stage.
Fig. 129.—The Abbe Condenser, detached from the Sub-stage of the Microscope.
The Abbe condenser is the most popular form in use, for all purposes. Owing to the large aperture of the cone of light which it projects, it can be employed with the highest powers; by removing the top lens it can also be used with low powers. Dark ground illumination may be obtained with it up to a ¼-inch objective.
Fig. 130.—Optical Arrangement of Abbe Illuminator, 1·2 N.A.
Fig. 131.—Optical Arrangement of Abbe Illuminator, 1·4 N.A.
The condenser is made in two forms of 1·2 and 1·4 numerical aperture by Messrs. Watson. The lenses are mounted in aluminium. [Fig. 130] is in more general use, but by workers with high powers [Fig. 131] is preferred, as it ensures the most oblique illumination with objectives of largest aperture. It is preferred for photo-micrographic purposes.
Fig. 132.—The Optical Arrangement of Watson’s Achromatic Condenser.
Watson’s Achromatic Condenser ([Fig. 132]), 1·0 numerical aperture, shown in section, although originally designed for use with the micro-spectroscope, is equally efficient for ordinary purposes. This condenser transmits a larger aplanatic cone of light than Abbe’s. It may therefore be employed with higher power objectives, and by removing the top lens it is just as useful a condenser for lower powers. Being constructed with lenses of an unusually large size, it is well adapted for use with the micro-spectroscope. It is certainly one of the best all-round condensers in use. The new Schott glass enters into the construction of the lenses, and these are mounted in aluminium.
Fig. 133.—Powell’s Achromatic Condenser.
Many microscopists consider on the whole that Powell’s sub-stage apochromatic condenser with collar correction ([Fig. 133]) surpasses that of Abbe. The mechanical arrangement of Powell’s is very simple: the correction collar is similar to that of an ordinary objective, it has a steeper spiral slot and only half a revolution of movement; a long arc is fixed to the collar so that it may conveniently be reached by the finger. It is so constructed as to turn easily and smoothly at the slightest touch. The collar moves only the back lens of the combination, leaving the mount rigid. The diaphragms are regulated by A and B.
 Fig. 134. |  Fig. 134a. |
 Fig. 134b.—Powell’s Apochromatic Oil Immersion Condenser, N.A. 1·40. | |
The object of the correctional movement is to increase the maximum aplanatic aperture of the condenser by separating the lenses. If the back of a wide-angled objective be examined when an object is illuminated by the full aperture of the condenser, the edge of the flame being in focus, it will be noticed that the illuminated portion of the back lens will be oval and pointed instead of circular. Also that when the condenser is racked up, although the external shape of the illuminated portion becomes more circular, two dark patches will appear on either side of the centre, showing the operation of the spherical aberration of the condenser. If under these circumstances the lenses are separated by means of the collar adjustment, the black spots will be closed up, and a circular and evenly-illuminated disc of illumination of a larger size will result. The wheel of diaphragms, or a series of graduated diaphragm discs to drop into a holder, is intended for critical work; the diaphragm can always be recorded, and the identical illuminating cone reproduced.
Hence we have a simple method of graduating apertures between any two contiguous diaphragms; if, for example, we place the lever to the left, so that the lens may be separated as far as possible, and use a No. 6 diaphragm, and if, on examining the object, it is thought that the illuminating cone is not large enough, and if when No. 7 is turned on it is found too much, we can go back to No. 6, and by turning the lever 60° towards the right, closing the lenses and increasing the power a little, we shall obtain an aperture somewhere between Nos. 6 and 7 diaphragm. Thus we can by means of the correction collar graduate the aperture with the facility as with an iris, and we can record any particular aperture with a degree of accuracy foreign to the iris. It must be admitted, however, that the cone of light transmitted by the condenser is a very small one.
Powell also supplies an apochromatic oil-immersion condenser, numerical aperture 1·40, but without collar correction; [Fig. 134] shows the sliding tube lowered by arm A and cell B withdrawn for changing stops, which can be done without altering the focus of the condenser. [Fig. 134]a shows the cell B closed and raised by arm A close to the back lens of optical combination. In [Fig. 134]b six of the principal stops are shown. Powell’s dry apochromatic condenser, of nearly 0·9 aplanatic cone, is also very good; but the high price of all is a bar to their more general use. The speciality of these is the conversion of axis light into condensed oblique incident light by the refraction of the condenser.
Messrs. R. & J. Beck have various forms of achromatic condensers, some of which partake of a somewhat elaborate arrangement; others are simple and inexpensive, to suit the students’ microscope; as when the light of the concave mirror proves insufficient for any object requiring intense transmitted light, an achromatic condenser must be adapted to even the students’ form of microscope. The latest form of condenser ([Fig. 135]) is fitted with revolving stops and iris diaphragm, and other appliances for obtaining satisfactory results.
Fig. 135.—Beck’s newer form of Achromatic Condenser.
Beck’s Compound Illuminating Apparatus ([Fig. 136]).—It is useful in working with the microscope to be enabled to rapidly change the illumination, and for this reason this compound form of condenser has been constructed. It consists of an upper portion A, a wide-angle condenser, the aperture of which can be reduced at will by an iris diaphragm, moved by the lever B. This can be used for all other purposes. Below this diaphragm is a plate C, which can be swung back out of position at will, as shown in outline. Into a cell in this plate the stops D can be dropped, and the condenser can be used for dark field illumination, or for high powers as an oblique illuminator. A large-size polarising prism E, fastens to the plate C, and can be removed when not required. In this way any of the various modes of illumination may be separately or conjointly obtained.
Fig. 136.—Beck’s Compound Condenser.
Fig. 137.—Beck’s Spherical Achromatic Condenser.
Their condenser ([Fig. 137]) has a large aperture, and facilities for rotating the series of diaphragms. It is available for either dry or immersion objectives up to 1·3 numerical aperture on diatoms, and wet or dry histological objects. The spherical form of the front is worked by a milled-head that rotates a series of lenses and diaphragms. It also avoids the inconvenience of having the connecting fluid drawn away by capillary attraction, as would be the case if mounted on a flat surface. It is also less in the way of the sub-stage movements.
Fig. 138.—Watson’s Parachromatic Condenser.
The Parachromatic Condenser of Messrs. Watson ([Fig. 138]) was made to meet a demand for a condenser giving a large solid cone of illumination free from colour. The optical part of this condenser consists of a full hemispherical front lens, and the middle and back combinations of such forms as to produce the necessary corrections. The Jena phosphate crown and silicate flints are used in its manufacture, and to these are due its special qualities. The total aperture of the condenser is 1·0, and it yields an aplanatic aperture of ·90 numerical aperture. The magnifying power is 2⁄7ths of an inch. From this it will be seen that it is especially suitable for use with high-power objectives.
It can also be employed without the front lens, when the magnifying power is 4⁄10ths of an inch, and the numerical aperture ·35. It is mounted in an exceedingly convenient manner, the iris diaphragm being fitted in such a way as to be absolutely central with the optical system.
The arc through which the handle controlling the iris travels is divided, and indicates the aperture at which the condenser may be working at any time. An important feature in this condenser is that it is almost wholly free from colour. As a rule condensers of the same form are found difficult to work with, because of the small diameter of the field or back lens. This difficulty has been successfully overcome by increasing the size of this lens, and the whole of which is fully utilised.
Most London opticians have their own especial form of achromatic condenser, designed for and fitted to their several stands and objectives, varying from a small price to the more expensively-fitted accessories.
Fig. 139.—Swift’s Illuminating Polarising Apparatus.
Fig. 139a.—Swift’s Diaphragms and Central Stops.
Messrs. Swift’s illuminating apparatus ([Fig. 139]) is conveniently supplied with numerous useful appliances. The optical combination A is computed to be used as an effective spot lens from a 3-inch objective up to a sixth. C C are two small milled heads by means of which the optical combination A is centred to the axis of the objective. The revolving diaphragm E has four apertures for the purpose of receiving central stops, oblique light discs, and selenite films. D is a frame carrying two revolving cells, into one of which a mica film is placed, which can be revolved with ease over either of the selenites below, whereby changes of colour can be obtained in experimenting with polarised light. The darts and P A’s indicate the position of the positive axis of the mica and selenite films, and by this means results can be recorded, etc. Either of the revolving cells can be thrown into the centre of the condenser, and there stopped by means of a spring catch; when so arranged the mica film, &c., may be revolved in its place by turning the cell D, as both cells are geared together with fine racked teeth. F is a polarising prism mounted on an eccentric arm, rendered central when in use, or thrown out, as seen, when out of use. G is the rack dove-tail slide for indicating and focussing the condenser on the object. The advantages associated with this condenser consist in having the polarising prism, selenite films, dark-ground, and oblique light stops, so that they may be brought close under the optical combination.
Fig. 140.—Baker’s Nelson Achromatic Condenser.
Baker’s Nelson Condenser, shown in [Fig. 140], is intended for use with their medium instruments. It has, however, many pieces of apparatus essential to those of a higher class. It is applicable, indeed, to all instruments having sufficient depth beneath the stage to receive it. It comprises an achromatic combination of 90° aperture, available with all powers up to 1⁄8-inch tinted glass for neutralising the yellow rays of artificial light, focussing adjustment, dark-ground illuminator, large diaphragm with rotating tube to carry oblique light stops, small wheel of apertures, polarising prism with two selenite films, clear aperture, and oblique light-shutter for low powers.
Baker’s Students’ Condenser ([Fig. 141]) is designed to take the place of Abbe’s, and costs much less. It transmits a larger aplanatic cone of light, and can be used either with high or low powers by removing the front lens. It is equally useful for photo-micrographic work.
Mr. J. Mayall’s semi-cylinder or prism for oblique illumination ([Fig. 142]) is a convenient form, as it permits of the semi-cylinder being tilted and placed excentrically; in this manner, without immersion contact, and by suitable adjustment, a dry object can be viewed with any colour of monochromatic light. If placed in immersion contact with the slide, the utmost obliquity of incident light can be obtained. Objects in fluid may be placed on the plane-surface of the semi-cylinder, and illuminated by ordinary transmitted light, or rendered “self-luminous” in a dark field, as with the hemispherical illuminator or Wenham’s immersion paraboloid. A concave mirror with a double arm is quite sufficient to direct the illuminating pencil. This semi-cylinder was originally made by Tolles, of Boston, for measuring apertures, but, at Mr. Mayall’s suggestion, Messrs. Ross mounted it as an illuminator.
Fig. 141.—Optical Arrangement of Baker’s Abbe Condenser.
The spiral slot should be fixed close beneath the larger lens of the condenser, and when properly arranged will be found a convenient mode of obtaining oblique light.
Fig. 142.—Mayall’s Semi-Cylinder Illuminator and Spiral Diaphragms.
The Webster-Collins Universal Condenser ([Fig. 143]) is so well known that it scarcely calls for any lengthy description. It is an inexpensive form of condenser, designed in the first instance for use with the students’ microscope. It is fitted into the sub-stage; has an iris diaphragm as well as a series of revolving diaphragms moved by a milled head screw arrangement.
Fig. 143.—The Webster-Collins Universal Condenser.