NE^W PROPERTY OF REFLECTED LIGHT. ^7 



production to employ two crystals of the same kind. Thus 

 the second crystal, for example;, may be carbonate of lead, 

 or sulphate of barytes ; the first may be a crystal of a sul- 

 phur, and the second of rock crystal. All these substances 

 comport themselves with one another in the same manner 

 as two rhomboids of calcareous spar. In general this pro- 

 pensity of light to be refracted in two pencils, or in one only, 

 depends solely on the respective positions of the axis of the 

 integrant particles of the crystals employed, be their chemi- 

 cal principles what they may, and of the natural or artificial 

 faces, on which the refraction is produced. This result 

 proves, that the modification lisrht receives from these dif- 

 ferent substances is perfectly identical. 



To render the phenomena I have described more sensi- Method of 

 ble, the flame of a taper may be viewed through two prisms rendering the 

 »f different substances, possessing the property of double more'evident. 

 refraction, placed on each other. In general we shall per- 

 ceive four images of the flame : but, if we turn one of the 

 prisms slowly round the visual ray as an axis, the four 

 images will be reduced to two, as often as the princi- 

 pal sections of the contiguous faces become parallel, or 

 cut each other at right angles. The two images that dis- ' 

 appear do not lose themselves in the other two ; we perceive 

 them gjfeduaily become extinct, while the other acquire 

 increasea intensity. When the two principal sections are 

 parallel, one of the images is formed by rays refracted 

 in the ordinary way by the two prisms, and the other by 

 rays refracted extraordinarily. When the two principal 

 sections are perpendicular, one of the images is formed by 

 rays refracted ordinarily by the first crystal, and extraordi- 

 narily by the second ; and the other by rays refracted 

 extraordinarily by the first crystal, and ordinarily by the 

 second. 



Not only all crystals, that double images, are capable of Light affecte 



giving light this faculty of being refracted in two pencils, '" a similar 



or in one only, according to the position of the refractive transpareat 



crystal ; but all transparent bodies, whether solid or liquid, ^'^^^^^j 



and even opake bodies themselves, can impress on the lumi- ^"'^^'<^" ^T 

 • , , . - , T • • 1 • , , , opake ones. 



nous particles this smgnlar disposition, which seemed to be 



one of the effects of double refraction. 



Vol. XXX.— Oct. 18U. ^ When 



