182 COLORED IMAGES. SECT. XXII. 



ance is presented. The cloudy spot in the direction of 

 the optic axis is seen surrounded by a set of vividly 

 colored 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 colored 

 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 colored circular rings, with a broad 

 black cross passing through its center, dividing the rings 

 into four equal parts. When the analyzing plate re- 

 volves, this figure recurs at eveiy quarter revolution ; 

 but in the intermediate positions it assumes the com- 

 plementary colors, the black cross becoming white. 



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

 nomena exhibited by innumerable bodies, which undergo 

 periodic changes in form and color when the analyzing 

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

 color without the aid of tourmaline or something equiv- 

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

 beautiful phantoms into existence. Tourmaline has the 

 disadvantage of being itself a colored substance ; but 

 that inconvenience may be obviated by employing a re- 

 flecting 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 colored pencils; and 

 when the analyzing plate is turned round in its own 

 plane, it will alternately reflect each ray at every quar- 

 ter revolution, so that all the phenomena that have been 

 described will be seen by reflection on its surface. 



Colored 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. 

 &c. In short, all the phenomena of colored rings may 

 be produced, either permanently or transiently, in a 

 variety of substances, by heat and cold, rapid cooling, 



