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.
