why should we use canadabalsem to glued lenses
Canada balsam 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.
Endoscopy Basil Hirschowitz and Larry Curtiss invented the first fiber optic endoscope in 1957.[20] Earlier in the 1950s Harold Hopkins had designed a "fibroscope" consisting of a bundle of flexible glass fibres able to coherently transmit an image. This proved useful both medically and industrially, and subsequent research led to further improvements in image quality. Further innovations included using additional fibres to channel light to the objective end from a powerful external source, thereby achieving the high level of full spectrum illumination that was needed for detailed viewing, and colour photography.[citation needed]