106 



THE MICBOSCOPK 



fig. 70. 



can always cleave it or split it into shapes represented by fig. 70, 

 wliich is called a rhomb of Iceland spar, a solid bounded by six equal 

 and similar rhomboidal surfaces, whose sides 

 are parallel, and whose angles hac, acd, are 

 101° 55' and 78° 5'. The line ax, called the 

 axis of the rhomb or of the crystal, is equally 

 inclined to each of the six faces at an angle 

 of 45° 23.' It is very transparent, and gene- 

 rally colourless. Its natural faces when it is 

 split are commonly even and perfectly polished ; 

 but when they are not so, we may by a new cleaverage replace the 

 imperfect face by a better one, or we may grind and polish an im- 

 perfect face. 



It is found that in all bodies where there seems to be a regularity 

 of structure, as salts, crystallised minerals, on light jiassing through 

 them it is divided into two distinct pencils. If we take a crystal of Ice- 

 land spar, and look at a black line or dot on a sheet of paper, there 

 will appear to be two lines or dots; and on turning the spar round, 

 these objects will seem to turn round also ; and twice in the revolution 

 they will fall upon each other, which occurs when the two positions of 

 the spar are exactly opposite, that is, when turned one-half from the 

 position where it is first observed^ In the accompanying diagram, fig. 7 1, 



the line appears double, as a 6 

 and c d, or the dot, as e and f. 

 Or allow a ray of light, g h, to 

 fall thus on the crystal, it will 

 in its passage through be se- 

 parated into two rays, hf, he; 

 and on coming to the opposite 

 surface of the crystal, they will 

 pass out at ef in the direction 

 of ih, parallel to gh. The 

 plane Imno is designated the 

 principal section of the crystal, 

 and the line drawn from the solid angle I to the angle o is where the 

 axis of the crystal is contained ; it is also the optic axis of the mineral. 

 Now when a ray of light passes along this axis, it is undivided, and 

 there is only one image ; but in all other directions there are two. 



If two crystals of Iceland spar be used, the only difference will be, 

 that the objects seem farther apart, from the increased thickness. But 

 if two crystals be placed with their principal sections at right angles to 



fig. n. 



