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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 

 different 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 arable, gold leaf placed on wax, tin foil, fusible metal, and realgar; 

 and, by meansof pressure, withlead; the appearances exhibited by these 

 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 difference of transparency ; 

 and the refractive density of the substance employed for the impres- 

 sion does not appear to have influenced the magnitude of the disper- 

 sion, 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 phe- 

 nomena 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 al- 

 ways 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 



