A mechanism is described that enables a specimen to be tilted through a known angle and continuously observed under the highest power of the optical microscope. Objects can thus be localized accurately in space. This facility in conjunction with serial optical sectioning by Nomarski optics has been used to construct models of the arrangement of polytene chromosomes in nuclei of Drosophila, Simulium and Chironomus. Telomeres and chromocentre lie on the nuclear envelope. In Simulium they lie close to an equator of the nucleus. Despite these constraints, different nuclei in the same gland do not resemble each other closely.
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