Criticalillumination

DENHAM, H. Microscope Illumination and Fatigue. Nature 108, 369–370 (1921). https://doi.org/10.1038/108369b0

Köhlerillumination

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A VERY large part of the fatigue produced by working at the microscope for long hours is due to the use of incorrectly adjusted illumination, whether too bright or too weak. In routine work of a critical nature, where daylight is out of the question for several reasons and an artificial light source must be used, the light may be regulated so as to be satisfactory for the combination of eyepiece and objective most frequently used, but any change of either materially alters the brilliancy of the field. Any alteration of the substage to correct for this will upset the critical adjustment of the optical system; the changing of light-filters is a rough-and-ready solution, but fine differences cannot easily be made without varying the quality of the light, and, in most cases, require the removal of the eye from the microscope to carry out.