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. Most London opticians have their own especial form of achromatic condenser, designed for and fitted to their several stands and objectives, varying from a small price to the more expensively-fitted accessories. [Illustration: Fig. 139.--Swift’s Illuminating Polarising Apparatus.] [Illustration: Fig. 139_a_.--Swift’s Diaphragms and Central Stops.] Messrs. Swift’s illuminating apparatus (Fig. 139) is conveniently supplied with numerous useful appliances. The optical combination A is computed to be used as an effective spot lens from a 3-inch objective up to a sixth. C C are two small milled heads by means of which the optical combination A is centred to the axis of the objective. The revolving diaphragm E has four apertures for the purpose of receiving central stops, oblique light discs, and selenite films. D is a frame carrying two revolving cells, into one of which a mica film is placed, which can be revolved with ease over either of the selenites below, whereby changes of colour can be obtained in experimenting with polarised light. The darts and P A’s indicate the position of the positive axis of the mica and selenite films, and by this means results can be recorded, etc. Either of the revolving cells can be thrown into the centre of the condenser, and there stopped by means of a spring catch; when so arranged the mica film, &c., may be revolved in its place by turning the cell D, as both cells are geared together with fine racked teeth. F is a polarising prism mounted on an eccentric arm, rendered central when in use, or thrown out, as seen, when out of use. G is the rack dove-tail slide for indicating and focussing the condenser on the object. The advantages associated with this condenser consist in having the polarising prism, selenite films, dark-ground, and oblique light stops, so that they may be brought close under the optical combination. [Illustration: Fig. 140.--Baker’s Nelson Achromatic Condenser.]