The microscopist : a manual of microscopy and compendium of the microscopic science : micro-minerology, micro-chemistry, biology, histology, and pathological histology / by J.H. Wythe.

Joseph Henry Wythe

Date:: 1877

Credit: The microscopist : a manual of microscopy and compendium of the microscopic science : micro-minerology, micro-chemistry, biology, histology, and pathological histology / by J.H. Wythe. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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$dered dark, and doubly refracting bodies on the stage of the microscope appear either illuminated or in colors. If a polarized ray passes through a doubly refracting film, as of selenite, it forms two distinct rays, the ordinary and the extraordinar}^ ray. Each of these will be of dif- ferent colors, according to the thickness of the film. If one be red, the other will be green, these colors being complementary. By using the analyzer one of these rays is alternately suppressed, so that on revolving the appa- ratus the green and red rays appear to alternate at each quarter of a circle. Films of selenite are often mounted so as to revolve between the polarizer and the stage. Barker's selenite stage is sometimes used for this purpose (Fig. 24). With such a stage a set of selenites is usually Diuker's S^'lenite Stusre. supplied, giving the blue, purple, and red, with their com- plementary colors, orange, yellow, and green. By this combination all the colors of the spectrum may be ob- tained. The selenite disks generally have engraved on them the amount of retardation of the undulations of white light, thus: J, f, and \. If these are place<] so that their positive axes (marked PA) coincide, they give the sum of their combined retardations. The Mic?'ospectrosco2)e.^—Ordimiry spectrum analysis, by determining the number and position of certain narrow lines in the spectra of luminous bodies, called Fraunhofer's$