The simplest possible microscope lamp
by Martin Canny, Australia
I have been frustrated over many years by the inadequacy of the lamps available to illuminate the field of a standard microscope equipped with an Abbe condenser and a mirror. They have varied from kerosene flames, through pearl bulbs in tin cans, coiled filaments, Cooke ribbon filaments, with various arrangements of condenser lenses, ground-glass surfaces, holes or diaphragms, to provide critical illumination, Köhler illumination or just some brightness of field. All were more or less unsatisfactory in providing uneven intensities across the field, and/or failing to fill the field of a low-power objective. Many were difficult to align and took up much bench space. The best was a Singer lamp that had two condenser lenses of different focal lengths.
I have discovered by chance that an even field of any size can be achieved with an absolute minimum of equipment. Place a sheet of white paper on the bench under the base of the microscope, extending well forward of the base. Light it with a bright reading lamp (e.g., 50 W halogen). Look down the microscope using any objective and turn the mirror until it reflects the bright paper. The illuminated field is bright and uniform, from oil immersion to a 41-mm Luminar objective (5 mm field of view). It works well with a dark field stop below the condenser. To fill the field (16 mm) of a Zeiss Plan x1 objective it is necessary to remove both the mirror and the condenser. I provide a few images taken with this illumination. The glare to the eyes direct from the paper can be easily blocked by judicious placing of lamp housing.
Comments to the author Martin Canny are welcomed.
Figs 1 - 4. Bright field micrographs of a transverse section of the wood of a young stem, of Fraxinus with the paper illumination and different Olympus objectives. Fig. 1 x4/0.10 objective; Fig 2 x10/0.25 objective; Fig. 3 x40/0.65 objective; Fig. 4 oil immersion x100/1.30 objective. Figs 5, 6 Dark field micrographs of a preparation of diatoms with the x4 and x40 objectives, respectively.
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