Astronomical and Nautical Collections* 1 69 



tioiij a determinate position, which is always the same for the 

 same crystal, however it may be presented to the incident rays. 



There are some crystals in which the perfect resemblance 

 of all sides of the axis is not strictly observed, and in which 

 there are consequently two particular directions more or less 

 inclined to each, which are possessed of properties resembling 

 those which belong to a single axis in the simpler form of 

 the phenomenon : and these are called crystals with two axes ; 

 but we shall consider, in the first instance, crystals with one 

 axis only, the optical properties of these being simpler and 

 more easily understood. 



A plane drawn through the axis, perpendicularly to the 

 surface of the crystal, is called its principal section. The 

 present object not requiring an explanation of all the differ- 

 ent manners in which the rays of light are bent by the crys- 

 tals, but merely of their mode of propagation in these me- 

 diums, and the optical properties which they acquire in them, 

 we may suppose, for the sake of simplicity, that the incident 

 rays are always perpendicular to the surfaces of the crystal, 

 and contained in the plane of its principal section : and when 

 it becomes necessary to study their progress in different di- 

 rections with respect to the axis, we may imagine in each case 

 that the surfaces of their admission and emersion are made 

 perpendicular to these directions. 



This being premised, we may observe, in the carbonate of 

 lime, which has a very conspicuous double refraction, that one 

 of the two pencils becomes oblique to the surface when the in- 

 cident light is perpendicular ; while the other proceeds with- 

 out being bent, in the manner of ordinary refraction: and this 

 ray is considered as ordinarily refracted, and the former ex- 

 traordinarily : the pencils are also called respectively ordi^ 

 nary and extraordinary ; and the images, which they form, 

 ordinary and extraordinary images. A similar bifurcation 

 takes place under the same circumstances in other doubly 

 refracting crystals, such as rock crystal, but the separation 

 is so slight that a considerable thickness is required to render 

 it sensible. It becomes more easily observable, when the 

 crystal is so cut, that the surface of emersion is inclined to 

 that of admission, which causes the two pencils to emerge at 



