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.
Fig. 139.—Swift’s Illuminating Polarising Apparatus.
Fig. 139a.—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.
Fig. 140.—Baker’s Nelson Achromatic Condenser.
Baker’s Nelson Condenser, shown in [Fig. 140], is intended for use with their medium instruments. It has, however, many pieces of apparatus essential to those of a higher class. It is applicable, indeed, to all instruments having sufficient depth beneath the stage to receive it. It comprises an achromatic combination of 90° aperture, available with all powers up to 1⁄8-inch tinted glass for neutralising the yellow rays of artificial light, focussing adjustment, dark-ground illuminator, large diaphragm with rotating tube to carry oblique light stops, small wheel of apertures, polarising prism with two selenite films, clear aperture, and oblique light-shutter for low powers.
Baker’s Students’ Condenser ([Fig. 141]) is designed to take the place of Abbe’s, and costs much less. It transmits a larger aplanatic cone of light, and can be used either with high or low powers by removing the front lens. It is equally useful for photo-micrographic work.