A resinous material produced by balsam fir trees. This material, when dissolved in xylene solvent, results in a transparent glue that can be used to assemble camera lenses - specifically, making optical groups - since it has nearly the same refractive index as glass.
Older lenses used Canada balsam as an optical cement. Sadly, balsam tends to yellow with age, making such lenses unsuitable for colour photography. (though the yellow tinting is often a desirable effect in black and white photography) This yellowing is independent of a similar problem with radioactive thorium-containing glass.
cf. cemented element, group, radioactive glass, refractive index.
Entry last updated 2002-04-26. Term 181 of 1487.
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