DOUBLE REFRACTION. 



double refraction in passing through 

 the rhomb, and as the very same phe- 

 nomena will take place by making the 



summit A upon any point of any of the 

 faces, it is manifest that the ' douhle 

 refraction cannot arise from any differ- 



ray Rr fall at the same incidence and ence of density in different parts of the 

 in the same direction relative to the rhomb. 



Fig. 2. 



In order to prove this, however, by 

 direct experiment let the angles of re- 

 fraction of the ray r o, r e be measured 

 corresponding to the different angles of 

 incidence of R r, beginning at a perpen- 

 dicular incidence or 0. It will then be 

 found that at the ray r o has suffered 

 no refraction, and that at 1 20 30, 

 &c. its refraction is such as it should be 

 by the ordinary law of the sines ; the 

 sine of the angle of refraction being to 

 the sine of the angle of incidence in a 

 constant ratio. With the ray r e, how- 

 ever, the case is very different ; at its 

 angle of refraction, in place of being 0, 

 is 6 12'; and at 10 20 30, &c. it is 

 such as not to follow the constant ratio 

 of the sines. Hence it follows that Ice- 

 land spar has a double refraction, sepa- 

 rating a pencil of light into two, one of 

 which is refracted according to the or- 

 dinary law, and the other according to 

 a new or extraordinary law. 



Def. 1. The ray ro is therefore called 

 the ordinary ray, and r e the extraor- 

 dinary ray. 



The property of double refraction is 

 possessed by a very great number of 

 minerals and artificial salts. It is found 

 also in various animal and vegetable 

 bodies, and it may be communicated 

 either transiently or permanently to 

 substances in which it does not naturally 

 reside. 



Def. 2. In all doubly refracting sub- 

 stances there are one or more lines, or 

 one or more planes, along which there 

 is no double refraction, or along which 

 no doubly refracting force exists. 



Def. 3. Those substances in which 



there is only one such line or plane, are 

 called crystals or bodies with one axis, 

 or one plane of oxen of double refrac- 

 tion, and those which have two, three, 

 four, &c. such lines are called crystals 

 or bodies with two, three, four, &c. 

 axes, or planes of axes, of double re- 

 fraction. 



Def. 4. When the doubly refracting 

 force does not exist in any of these axes 

 or planes of axes, the axis is called a 

 real axis of double refraction ; but 

 when the disappearance of double re- 

 fraction arises from the existence of 

 two opposite and equal doubly refract- 

 ing forces which destroy one another, 

 the axis or plane of axes are called a 

 resultant axis or plane of double re- 

 fraction, or an axis or plane of compen- 

 sation. 



Def. 5. If the ray which suffers the 

 extraordinary refraction is refracted 

 towards the axis or plane of axes of a 

 doubly refracting body, the axis is called 

 a positive axis of double refraction; 

 and if it is retracted from the axis, it is 

 called a negative axis of double refrac- 

 tion. 



CHAPTER I. 



Crystals with one Axis of Double Re- 

 fraction List of such Crystals 

 Law of Double Refraction in those 

 with a Negative Axis Law of it in 

 those with a Positive Axis Crys- 

 tals with two Axes of Double Re- 

 fraction Crystals with one Axis 

 for one Coloured Light, and two for 

 another Coloured Light Crystals 

 with many Planes of Double Refrac- 

 tion Circular Double Refraction 



