SECT, xxn.] COLOURED IMAGES. 211 



at such an inclination as to transmit the polarized ray along 

 one of its optic axes, is viewed through the tourmaline with 

 its axis vertical, a most splendid appearance is presented. 

 The cloudy spot in the direction of the optic axis is seen 

 surrounded by a set of vividly coloured rings of an oval 

 form, divided into two unequal parts by a black curved 

 band passing through the cloudy spot about which the rings 

 are formed. The other optic axis of the mica exhibits a 

 similar image (N. 207). 



When the two optic axes of a crystal make a small angle 

 with one another, as in nitre, the two sets of rings touch 

 externally; and, if the plate of nitre be turned round in its 

 own plane, the black transverse bands undergo a variety of 

 changes, till, at last, the whole richly coloured image assumes 

 the form of the figure 8, traversed by a black cross (N. 208). 

 Substances with one optic axis have but one set of coloured 

 circular rings, with a broad black cross passing through its 

 centre, dividing the rings into four equal parts. When 

 the analyzing plate revolves, this figure recurs at every 

 quarter revolution ; but in the intermediate positions it as- 

 sumes the complementary colours, the black cross becoming 

 white. 



- It is in vain to attempt to describe the beautiful pheno- 

 mena exhibited by innumerable bodies, which undergo 

 periodic changes in form and colour when the analyzing 

 plate revolves, but not one of them shows a trace of colour 

 without the aid of tourmaline, or something equivalent, to 

 analyze the light, and as it were to call these beautiful 

 phantoms into existence. Tourmaline has the disadvantage 

 of being itself a coloured substance ; but that inconvenience 

 may be obviated by employing a reflecting surface as an 

 analyzing plate. When polarized light is reflected by a 

 plate of glass at the polarizing angle, it will be separated 

 into two coloured pencils ; and, when the analyzing plate is 

 turned round in its own plane, it will alternately reflect each 

 ray at every quarter revolution, so that all the phenomena 



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