Canada balsam

[1] 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.

Other optical elements can be cemented with Canada balsam, such as two prisms bonded to form a beam splitter.

[3] Some workers prefer terpene resin for slide mounts, as it is both less acidic and cheaper than balsam.

Calcite is an anisotropic crystal and has different refractive indices for rays polarized along directions parallel and perpendicular to its optic axis.

Synthetic resins have largely replaced organic balsams for use in slide mounts.

The balsam fir tree from which Canada Balsam is obtained is so named because it is found almost entirely within Canada in the boreal forest region.
Slide of a holotype specimen of a flatworm ( Lethacotyle fijiensis ) permanently mounted in Canada balsam in 1953