3 H SCIENTIFIC APPARATUS. 



polishing has to be effected by hone-stones, such as " Water of 

 Ayr" stone, succeeded by Turkey-stone or Kansas-stone. The 

 polishing material, however, which has to be employed for finish- 

 ing the surface varies with the hardness and texture of the sub- 

 stance, and to this experience is the only guide. The friction of 

 the hand, however, under a stream of water generally, where the 

 material is not too refractory, leaves the polish the most complete, 

 and in the case of a rock section maintains the cleanest surface. 



Supposing, then, that we are enabled thus to present the crystal 

 in any required form to the process of investigation, we may con- 

 sider the general character of the instruments by which we are to- 

 examine it. Historically we should have to commence with the 

 polariscopes employed by Biot and Brewster in the second and 

 third decades of the century, or the original instrument of Norren- 

 berg, and those of Dove, Amici, and Soleil, while in variety they 

 would range from the simple tourmaline tongs of the lecture room 

 to the modern polarising microscope of Norrenberg, or the instru- 

 ment of Amici as developed in the able hands of Des Cloizeaux. 

 We may be content, however, with alluding only to the more 

 refined sorts of instrument. 



The stauroscope of Von Kobell had for its object the determi- 

 nation of the directions of the optical principal sections, on dif- 

 ferent faces, of crystals, and a means of distinguishing the system 

 to which a crystal might belong. The principle of the instrument 

 is founded on the distortion introduced into the stauroscopic 

 figure presented by an equatorially-cut plate of calcite (seen by 

 somewhat convergent light between two crossed Nicols or tourma- 

 lines), when a transparent crystal is placed in the path of the rays 

 in such a position as that its principal sections are not coincident 

 in direction with the axes of the tourmalines (or principal sections 

 of the Nicols). A rotation of the crystal into a position in which 

 this coincidence is established, gives the means of measuring the 

 angle at which its apparent principal sections are inclined on 

 some edge or crystallographic direction, previously determined on 



