372 Intelligence and Miscellanies. 



and these are, generally, crystallized bodies, or such as have 

 their particles symmetrically arranged. 



It must however be observed, that some doubly refracting 

 bodies, possess that property in so slight a degree, as not to 

 be sensible to ordinary direct observation. 



It is also found that the property o{ double refraction^ does 

 not exist equally, in every direction in the same crystal. But 

 that there is one direction in some crystals, and in others 

 there are two directions, in which if light be perpendicularly 

 incident, upon planes perpendicular to those directions, no 

 division of the ray takes place in its passage through ihe. 

 crystal. These directions are termed the optic axes of crys- 

 tals, or their axes of double refraction. When double re- 

 fraction takes place in every direction but one, the crystal is 

 said to possess a single axis. When there are two direc- 

 tions in the same crystal, in which double refraction does not 

 take place, the crystal is said to possess two axes ; and these 

 are always found to lie symmetrically on each side of some 

 particular crystallographical line within the crystal. 



Hole 3 of c/, is covered with a specimen of carbonate of 

 lime, and hole 4 with one of quartz, each of these having 

 only a single axis, and both being cut perpendicular to that 

 axis. Hole 5 is covered with a specimen of nitre, and 6 with 

 one of arragonite, both of which have two axes of double 

 refraction. 



Fig. 4, is intended to shew the relative positions of the 

 axes, in difl'erent crystals, with one and two axes. When a 

 crystal has only a single axis, its optical character may be 

 observed by looking perjyendicularly through the instrument, 

 as in the direction e f But where there are two axes, these 

 may lie near together, as gf f g, or they may be further 

 separated, as at ///, hf Where the angle gfg is small, 

 as in nitre, the phenomena of the two axes may be observed 

 at the same time; but where the angle, as hfli, is greater, 

 as it is in arragonite, the instrument must be held aslant, 

 fust on one side, and then on the other, to observe the phe- 

 nomena of both the axes. 



The phenomena exhibited may be varied in an interesting 

 and beautiful manner, by turning round either the moveable 

 part, fig. 3, by means of the external pin, or the tube a, or by 

 varying the relative positions of both. 



As an investigation of these phenomena would involve 

 some of the most abstruse and difficult problems in the the- 

 ory of light, it will not be attempted here, but the reader is 



