In optics, a (usually) triangular block of transparent material (plastic, glass, or silica) commonly used to bend a ray of light or split a light beam (for example, white light) into its component colours. Prisms are used as mirrors to define the optical path in binoculars, camera viewfinders, and periscopes. The dispersive property of prisms is used in the spectroscope.
The white saltire comes from the flag of Scotland. The St Patrick's Cross was, in fact, taken from the arms of the powerful Geraldine family. The red cross of St George is taken from the flag of England. Effective date: 1 January 1801.