Canada Balsam

Canada Balsam, an oleoresin obtained from the balsam secreted by the bark of the evergreen fir tree known as balsam fir. The balsam fir grows in the forests of Canada and the northern United States. The balsam exudes from the tree as a greenish-yellow liquid with the consistency of honey and becomes hard and transparent as it dries. Although balsam is sometimes an ingredient of varnishes, it is used chiefly as an adhesive for mounting microscopic specimens and in the manufacture of lenses and optical systems. Balsam has two important advantages when used to cement the elements of compound lenses and other glass surfaces: it is transparent, and its index of refraction is so close to that of optical glass that it introduces little distortion.