104 DOUBLE KEl'KACTION OF CRYSTALS. 



2. That these figures are altered, in proportion as we recede 

 from this ; and that at a great distance, when these objects are 

 seen under an angle of one or two minutes, they appear sur- 

 rounded with several irradiations, two of which are in the 

 direction of the eye-lids. 



3. That these irradiations are independent of the figure of 

 the luminous object, and are produced by the organ perceiving 

 them. 



4. That these irradiations are occasioned chiefly by the 

 irregular figure of the surfaces of the crystalline and cornea. 



5. Lastly^ that this irradiation is not well distinguished, 

 except in the dark ; because, the iris having then a greater 

 opening, the irradiation occasioned by the irregularity of the 

 surfaces of the crystalline and cornea becomes more percep- 

 tible. 



IV. 



On the doulle Refraction of Light in transparent Crystals : ly 

 Mr. Laplace.* 



Refraction of TT IGHT, when it passes from the air iato a transparent rae-< 

 '^ * * JLj dium not crystallized, is refracted so that the lines of inci- 



dence and refraction are constantly in the same ratio : but in pass- 

 ing through most diaphanous crystals it exhibits a singular phae- 

 nomenon, which was first observed in Iceland crystal, where it 

 is very perceptible. 

 DoubJe refrac- ^ ''^>' °^ ^'S'^'^' falling perpendicularly on one of the natural 

 tion. faces of this crystal, divides into two parts ; one traversing the 



crystal without altering its direction ; the other deviating from 

 it in a plane perpendicular to that face, and passing through the 

 axis of the crystal, that is, through the line that unites the sum- 

 mits of its two obtuse solid angles. This division of the ray 

 generally takes place with regard to any face, natural or artifi- 

 cial, and whatever be the angle of incidence; one portion fol-' 

 lowing the law of common refraction, the other a law of extra- 

 Law of the ex- ordinary refraction first discovered by Huygens ; and which, 

 traordinary re- considered as the result of experiment^ may be classed among 



fraction disco- 

 vered by Huy* 

 gene. ' * Journal des Mines, vol, XXIV, p. 401. 



th» 



