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.