Powell & Lealand Low Power Achromatic Condenser

摘要

The firm of Powell & Lealand was renowned in the Victorian period for striving for perfection in all aspects of the microscopes they produced. This included not only their objectives but also the substage condensers provided with their stands for the illumination of the specimens. It was recognised from the introduction of achromatic objectives, that a well-corrected condenser was needed for optimum image formation by precise control of the illumination. E M Nelson in his influential contribution to The Microscope and its Revelations [1], stressed the importance of freedom from spherical aberration in condensers, and that their aplanatic aperture should be as close as possible to their full aperture. In order to meet this need Powell Lealand introduced their achromatic condenser in 1857 with a numerical aperture close to unity, and produced an improved version in 1878 (Fig.1). Nelson [2] described this as 'the best ever introduced'. With the aim of further improvements P & L produced apochromatic condensers in 1891, added a correction collar to allow for variations in slide thickness in 1895 [3], and produced achromatic oil immersion condensers in1886 (Fig.2).