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. [Illustration: 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. [Illustration: Fig. 136.--Beck’s Compound Condenser.] [Illustration: 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. [Illustration: 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.