3'56 - On Cnjitallography , 



ininalion of il kite for those which reflect without order the 

 assemhiage of all the colours, like statuary inarhle. 



12. Dofthle refraction. When a ray of light passes 

 obliquely from one medium into another of a diHerent den- 

 sity, it is diverted from its route, forminir a kind of fold, 

 Ths deviation, which we call refraction, is subjected to a 

 constant law which is known to all naturalists. 



' Certain substances have the singular property to solicit 

 the ray which penetrates them to divide itself into two 

 parts which follow two different routes. This is called 

 doithle refraction. 



When the refraction is simple, we only perceive a single 

 image of an object seen through two faces of a transparent 

 piece of the mineral employed on this occasion, whereas, 

 if it were doable, we might in the same case see two images 

 of the object. But in order to obtain this eflect with most 

 of the substances endowed with the property in question, 

 we must choose two faces inclined towards each other, 

 whether we employ a crystal giveii by nature or a piec« 

 cut by the lapidary. 



The quantity of double refraction, or, what comes to the^ 

 same thing, the opening of the angle formed between each 

 other by the rays, by means of which the eye sees the two 

 images, varies froiTi one substance to the other, every thing 

 else being considered according to the nature of the sub- 

 stances themselves. In zircon, for instance, the double 

 refraction is very strong, whereas it is much less percepti- 

 ble in the eitierald. Besides, this quantity varies in every 

 substance, from various causes. In general it increases 

 or diminishes, according as the rcfrangent angle, or that 

 which is formed between each other by the two faces, 

 through which we view objects, is more oy less open. But 

 there is another cause of variation, which is combined 

 with the foregoing, and which depends on the position of 

 the refrangent surfaces relatively to the faces of the pri- 

 mitive form ; and such is the influence of this cause, that 

 under two equal refrangent angles difierently situated, we 

 may have distances evidently unequal between the imao:es 

 of the same object, and there is even a limit at which Fhe 

 effect of the double refraction becomes null, i. e. the two 

 images are then confounded into one. 



This limit takes place, for instance, in the quartz and in 

 the emerald, when one of the faces which belong to the 

 refrangent angle is perpendicular to the axis. It takes 

 place in sulphated barytes^ when one of the^anie faces being 



■^ -parallel 



