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. [Illustration: 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. [Illustration: 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. [Illustration: Fig. 134.] [Illustration: Fig. 134_a_.] [Illustration: Fig. 134_b_.--Powell’s Apochromatic Oil Immersion Condenser, N.A. 1·40.]