DOUBLE REFRACTION. 



found that all crystals which belong to 

 the prismatic system of Mohs, or whose 

 primitive forms are the right prism with 

 its base a rectangle, a rhomb, or an 

 oblique parallelogram ; the oblique 

 prism, with its base a rectangle, a 

 rhomb, or an oblique parallelogram, or 

 the rectangular and rhomboidal octa- 

 hedron, have two axes of double refrac- 

 tion. 



In these cases the double refraction 

 follows a very complicated law (see Chap. 

 VIIL); and M. Fresnel has made the 

 important discovery that both the rays 

 follow a law of extraordinary refraction. 



5. On Crystals which have two Axes 

 for the most refrangible, and one 

 Axis for the least refrangible rays. \ 

 This singular property was discovered 

 by Dr. Brewster in Glauberite, in which 

 he found two resultant axes inclined 

 to one another at an angle of 5 when 

 red light was used, and only one nega- 

 tive axis when violet light was used. In 

 this case, however, it may be shown, by 

 principles which will afterwards be ex- 

 plained, that glauberite has more than 

 one real axis even for the violet rays*. 



6. On Crystals with many Planes of 

 double Refraction. 



In all the crystals hitherto mentioned the 

 double refraction is related solely to one 

 or more lines or axes ; but Dr. Brewster 

 has found that ancdcime has its double 

 refraction related to various planes 

 within the crystal, in all of which the 



double refraction disappears. This re- 

 markable structure will be more parti- 

 cularly described in a subsequent part 

 of the treatise. 



7. On Crystals with circular double 

 Refraction. 



M. Fresnel has discovered [that a ray 

 of light passing along the axis of quartz 

 where its ordinary double refraction va- 

 nishes, is divided into two rays which have 

 remarkable properties. The law of varia- 

 tion of the doubly refracting force is 

 not known, but the properties of the 

 two rays to which it gives rise will be 

 afterwards described. 



8. On Bodies to which double Refrac- 

 tion may be communicated by Heat 

 and Pressure. 



Bodies with one or more axes of dou- 

 ble refraction may be formed artificially 

 out of glass, &c. either by pressure or 

 by the transmission of heat, or by rapid 

 cooling. In these cases the double re- 

 fraction depends on the external form 

 of the body, and changes with a change 

 of form. If the body is a cylinder, it 

 may be made to have one negative or 

 one positive axis of double refraction. 

 If it is a cylinder whose section is an 

 ellipse, or if it is a parallelepiped, it will 

 have more than one axis ; and if it is a 

 sphere it will have an infinite number of 

 axes of double refraction. In all these 

 cases the double refraction may be accu- 

 rately calculated, as will be shown in a 

 subsequent part of this treatise. 



PART II. 

 ON THE POLARISATION OF LIGHT. 



" THE Phenomena of the Polarisation 

 of Light," to use the language of one of 

 our most eminent mathematicians and 

 natural philosophers'!', " are so singular 

 and various, that to one who has only 

 studied the subject of physical optics 

 under its ordinary relations, it is like 

 entering into a new world, so splendid 

 as to render it one of the most delightful 

 branches of experimental inquiry ; and 

 so fertile in the views it lays open of the 

 constitution of natural bodies, and the 

 minuter mechanism of the universe, as 

 to place it in the very first rank of the 

 physico-mathematical sciences." 



When liirht emitted from the sun, or 

 from any self-luminous body, is reflected 

 from the surface, or transmitted through 



* Edinburgh Journal of Science, No. XIX. 

 j- Mr. Herschel, in his Treatise on Light. 



the substance of any homogeneous un- 

 crystallised body, the property of the 

 reflected or transmitted light continues 

 the same when we turn round the body, 

 so that the light falls on the first surface 

 always at the same angle ; that is, the 

 different sides of the rays exhibit no 

 different properties in relation to the 

 plane of its incidence. Such light is 

 called common light. 



A kind of light, however, has been 

 discovered which, when reflected from 

 the surface, or transmitted through the 

 substance of homogeneous uncrystal- 

 lised bodies, exhibits different proper- 

 ties when the body is turned round in 

 the manner above described. Hence 

 it follows that different sides of the rays 

 of such light must have different pro- 

 perties in relation to the plane of their 



