188 COLOURED IMAGKs. 8CI, xxil. 



ami, it' the |>late of nitre he turned round in its own plane, tin- 

 black transverse bands undergo a variety of changes, till, at last. 

 the whole richly coloured image assumes the form of the figure S, 

 traversed by a black cross (N. 213). 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 ligmv 

 recurs at every quarter revolution; but in the intermediate posi- 

 tions it. assumes the complementary colours, the black cross 

 becoming white. 



It is in vain to attempt to describe the beautiful phenomena 

 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 tonrmaline, 

 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 that have been 

 described will be seen by reflection on its surface. 



Coloured rings are produced by analyzing polarized light 

 transmitted through glass melted and suddenly or unequally 

 cooled ; also through thin plates of glass bent with the hand, jelly 

 indurated or compressed, &c. &o. In short, all the phenomena 

 of coloured rings may be produced, either permanently or tran- 

 siently, in a variety of substances, by heat and cold, rapid cooling^ 

 compression, dilatation, and induration ; and so little apparatus 

 is necessary for performing the experiments, that, as Sir John 

 Herschel says, a piece of window glass or a polished table to 

 polarize the light, a sheet of clear ice to produce the rings, and a 

 broken fragment of plate-glass placed near the eye to analyze the 

 light, are alone requisite to produce one of the most splendid of 

 optical exhibitions. 



Pressure produces remarkable changes in the optical properties 

 of crystals. Compression, perpendicular to the axis, transforms 

 a crystal with one optic axis into one with two. A slice of 



