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Literally 'algae seeker.' -A small simple
microscope of moderate power designed to determine if a given sample of
water contains diatoms or similar life forms. The term was first used
by Edourd Thum who sold such devices, of cylindrical form and of at
least moderate magnifying power.
A lens system used above a
specimen that has usually had polarized light projected through it.
This upper system is usually oriented 90 degrees to the polarizer, thus
blocking out all light rays not changed by the specimen itself. Without
an anisotropic specimen, the field will appear black when both analyzer
and polarizer are present and oriented 90 degrees from each other; this
is known as extinction. Analyzers can form part of a special eyepiece
(eyepiece analyzers), can take the form a removable extension of the
nosepiece (nosepiece analyzers), attach to the end of a draw tube, or be integrated into the main optical
tube, often with facility for throwing them out of the optical path.
ANGLE OF APERTURE: The angular aperture of a lens is the angular size of the lens aperture as seen from the focal point. It is an indication of the resolving power of a lens; the higher the angle of aperture, the better the resolving power. The angle of aperture reflects a different degree of resolving power depending on the medium. An angle of aperture of 75 degrees in air does not imply the same resolving power as 75 degrees in oil. Numerical aperture is a related term that compares resolving power regardless of the medium used.
ANIMALCULE(S): Microscopic life form(s), especially protozoa.
ANIMALCULE CAGE: see Live Box.
ANISOTROPIC: In microscopy, a material or
specimen which changes the orientation of polarized light and is
therefore visible between crossed polars. These materials are called birefringent.
A device for measuring the
numerical aperture of an objective. The Abbe apertometer consists of a
special calibrated scale with adjustment that sits on the stage, and a special objective that screws on to the inside of the drawtube of the microscope. Note that use of this type of apertometer requires both parts; an alternative to the special objective is to use a phase telescope or a Bertrand lens to visualize the back (top)focal plane of the objective. There were many types of apertometer developed. Other simpler variations include the Cheshire type, and the Beck type.
Part of the limb of a microscope that supports the main optical tube. This term is usually used to refer to the part which is extending forward from the rest of the limb to hold the optical tube.
The first synthetic plastic substance, was first made in the USA in 1907 and patented in 1909. In many respects Bakelite is superior to celluloid which preceded it. It was made from phenol and formaldehyde and came into wide use by 1910-25. It was sometimes used to make minor parts for microscopes like parts of eyepieces. The lens housing of the 'Seerite' tripod magnifier, made in the 1930's by the Testrite company, seen to the left, is made of Bakelite. Bakelite is highly resistant to solvents, is a very good insulator, and resists heat, but is very brittle and so breaks easily if dropped or subjected to physical pressure. In addition, extreme humidity can cause it to swell. Bakelite was used for the distributor cap for the early VW Beetle, and for a time even for Jewelry. It was replaced by less brittle plastics starting in the 1940's.