POWELL AND LEALAND APOCHROMATIC OIL IMMERSION CONDENSER

C. 1891

INVENTOR & MAKER: POWELL & LEALAND

MODEL: HIGH POWER OIL IMMERSION APOCHROMATIC CONDENSER(also available with achromatic optics)

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With the appearance of Powell & Lealand's achromatic oil immersion objectives with the very great apertures of 1.4 and 1.5, and their introduction of apochromatic objective systems, their chromatic oil immersion condenser was subsequently succeeded by an achromatic and then apochromatic condenser, on the same immersion principle, but out of necessity mechanically different, so it could be employed for transmitted, dark ground, and oblique illumination, and allow the use of different removable stops. To work most effectively, stops need to be placed close to the lower focal point of the condenser's optical parts, and inevitably, in high power condensers this is high up in the condenser tube near its lower lens element.As a result, the user was often forced to lower the condenser, thus bringing it out of focus, in order to change the diaphragm stops, when a different mode or angle of illumination was required, a significant disadvantage. To get around this problem, Powell & Lealand (at the instigation of Dallinger, see: Carpenter and Dallinger, 7th ed. (1891), page 254) devised a rising and falling sleeve which carried a swing-out holder for the diaphragm stops, which acted as an elevator, taking the stops up to the required position, so that they could be changed while the condenser remained focused on the slide. On this design, an achromatic version (N.A. 1.30)was constructed in 1886, and one on the apochromatic principle (N.A. 1.40) in 1891. Apart from being listed in Powell & Lealand's own catalogues, this condenser was also advertised by W. Watson & Sons in 1902: Powell & Lealand's Apochromatic oil immersion condenser, 1.4 N.A. mounted in P & L's special carrier, with stops, complete GBP 12 0 0 . Unfortunately, in this particular example of this rare device the front lens element was missing, which the owner has aimed to rectify by creating a replacement, which optically appears to work satisfactorily, save for resulting in a lower N.A.