<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>
why should we use canadabalsem to glued lenses
Due to its high optical quality and the similarity of its refractive index to that of crown glass ( n = 1.55 )