The eagle has always been a popular symbol in decoration and according to Joan Evans, op.cit. (1970) p.64 'For state occasions royal and noble persons were apt to wear brooches shaped as eagles, a traditional form which, it may be remembered is found as early as the eleventh century'. The idea of enclosing a devotional jewel in a resplendent outer frame was popular in Sicily. Cf. Di Natale, op.cit. I.11. For a fuller explanation of the imagery surrounding the Virgin see J.Hall, Dictionary of Subjects and Symbols in Art (1974)p.327 and for the imagery used in on a larger scale cf. L. Lee, G. Seddon, F. Stephens, Stained Glass (1976) p.32 for a 17th Century window in Troyes Cathedral