by reflection in the neighbourhood of the common image, having its
blue extremity towards this image, and being always situated, with
respect to it, in the direction of an axis of extraordinary reflection,
the angular distance varying with the inclination and situation of the
rays, and being also different in magnitude in different specimens,
but always observing certain laws. There is also a mass of coloured
light, crimson at great angles of incidence, and green at smaller, be-
yond the regular coloured image, its distance varying according to a.
diEerent law; becoming brighter when the substance is polished, and
varying also with its thickness. Similar appearances are observed in
a surface obtained by fracture; but a higher polish produces a. new
coloured image on the opposite side of the common image, and
nearly as bright as the former, which is rendered somewhat less bril-
liant by the operation of polishing. Similar appearances, but some-
what less distinct, are observed when a candle is viewed through a
thin piece of mother-of-pearl ; and it is remarkable, that the image
which is the brighter when seen by reflection, is the less bright when
seen by transmission. When the opposite surfaces happen to have
different axes of extraordinary reflection, they produce the appear.
ance of four images in the transmitted light.
Dr. Brewster having had occasion to fix a piece of mother-of-pearl, by a. hard cement, to a goniometer, was much surprised to find that the cement had acquired properties, with respect to colour, nearly similar to those of the mother-of-pearl; and he afterwards succeeded in producing the same effect with black and red wax, balsam of Tolu, gum arabic, gold leaf placed on wax, tin foil, fusible metal, and realgar; and, by meansof pressure, Withlead; the appearances exhibited bythese substances varying also, like that of the mother-of-pearl, from which they were derived, according to the degree of polish. But the mass of crimson and green light never accompanies these appearances : and, on the other hand, it is produced by mother-of—pearl, even when its reflection is destroyed by the contact of a substance of equal re- fractive density; so that it appears to depend on the internal consti- tution of the mother-of-pearl. The colours seen by transmission are more brilliant in gum arabic which has received the impression, than in the original substance, on account of the diEerence of transparency ; and the refractive density of the substance employed for the impression does not appear to have influenced the magnitude of the dispersion, as exhibited by the coloured images. Pearls also were found to communicate their properties to other substances in a similar manner, the principal image being surrounded with a. nebulosity, which is observable in an impression on wax.
Hence Dr. Brewster very naturally inferred that the peculiar phenomena of mother-of-pearl are owing to a particular configuration of its surface; and he had the satisfaction to find this inference fully confirmed by microscopical researches. The surfaces are almost always visibly grooved, so as somewhat to resemble the skin of the fingers: the grooves are sometimes perceptible to the naked eye, but sometimes too fine to be discovered even with magnifying powers
