This is situated immediately above the illumination system and below the condenser, although it is connected to the stage and the condenser by the same upright bracket.
Its function is to convert the natural light from the illumination system into plane polarised light.
The plane of vibration of the light in the polariser can be turned in some kinds of microscopes but in normal working conditions it is always fixed at 0°, often coinciding with what could be called the "East-West" direction.
The polariser is always positioned in the pathways of the light rays for the study of any optical property.
Polarised light is produced by the polariser and analyser, both of which in modern microscopes consist of a sheet of plastic (polaroid) which absobs all light except that vibrating in one direction. Older microscopes employed an ingeneous combination of calcite prism to produce polarised light (described first by W. Nicol and known as Nicol prisms).
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