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.