394 ON LIGHT. 



of the ordinary, and spheroidal of the extraordinary ray, 

 within them. In all or nearly all these, two lines in- 

 clined to one another at an angle greater or less accord- 

 ing to the nature of the substance can always be found 

 (either by a careful examination of the crystal in polar- 

 ized light through the faces of its natural form, or by 

 cutting and artificially polishing plates of it), which 

 possess the properties of such axes ; along which, that 

 is to say, refraction is single for a ray passing either way 

 out of the crystal; and in which when examined in 

 polarized light with an analyzing plate between the eye 

 and the crystal, coloured rings are seen. The simplest 

 and readiest instance of a crystal of this kind is furnished 

 by a sheet of ordinary mica, such a^ may easily be pro- 

 cured in large sheets. If a sheet of this be held before 

 the eye in a polarized field perpendicularly (an analyz- 

 ing plate being interposed) and turned round in its own 

 plane, two portions will be found at right angles to each 

 other, in which the polarization of the incident light is 

 not disturbed, and the field remains dark. Of the two 

 planes perpendicular to the plate in which the plane of 

 polarization cuts it in these two positions, one is the 

 " principal section " of the plate, and contains its " optic 

 axes." These may be brought into sight by holding the 

 eye (armed with the analyzer) quite close to the mica, 

 and inclining the latter, either forward or backward in 

 one of these two section-planes so as to make an angle 

 of about 35 with the visual ray on either side of the 

 perpendicular. In either situation a set of coloured 

 rings will be seen, not circular, but of an oval form, 



