WHEN a well-corrected oil-immersion objective is used to examine dust particles mounted in a medium such as Canada balsam, of similar refractive index to the oil, the finer particles will give a very similar image on either side of the correct focus, provided the objective is used at the tube-length for which it is designed. We have found, however, that if the dust deposit is mounted dry on the underside of the microscope cover-glass, the image on either side of the correct focus is not the same at the specified tube-length, and in order to obtain the best images, the tube-length must be reduced.
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BEADLE, D., PATTERSON, H. Effect of Tube-Length on the Visibility of Dust Particles with an Oil-immersion Objective. Nature 144, 327 (1939). https://doi.org/10.1038/144327a0
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