<P> Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine made from the resin of the balsam fir tree (Abies balsamea) of boreal North America . The resin, dissolved in essential oils, is a viscous, sticky, colourless or yellowish liquid that turns to a transparent yellowish mass when the essential oils have been allowed to evaporate . </P> <P> Canada balsam is amorphous when dried . Since it does not crystallize with age, its optical properties do not deteriorate . However, it has poor thermal and solvent resistance . </P> <P> Due to its high optical quality and the similarity of its refractive index to that of crown glass (n = 1.55), purified and filtered Canada balsam was traditionally used in optics as an invisible - when - dry glue for glass, such as lens elements . Lenses glued with Canada balsam (or with other similar glues) are called cemented lenses . Also, other optical elements can be cemented with Canada balsam, such as two prisms bonded to form a beam splitter . Balsam was phased out as an optical adhesive during World War II, in favour of polyester, epoxy, and urethane - based adhesives . In modern optical manufacturing, UV - cured epoxies are often used to bond lens elements . </P> <P> Canada balsam was also commonly used for making permanent microscope slides . From about 1830 molten Canada balsam was used for microscope slides, then Canada balsam in solution was introduced in 1843, becoming popular in the 1850s . In biology, for example, it can be used to conserve microscopic samples by sandwiching the sample between a microscope slide and a glass coverslip, using Canada balsam to glue the arrangement together and enclose the sample to conserve it . Xylene balsam, Canada balsam dissolved in xylene, is also used for preparing slide mounts . Some workers prefer terpene resin for slide mounts, as it is both less acidic and cheaper than balsam . Synthetic resins have largely replaced organic balsams for such applications . </P>

Why should we use canadabalsem to glued lenses