344 On the Methods of cutting 



rock crystal, in the positions requisite to produce, by their com- 

 bination, the double effect to which I allude. 



There are three principal directions in which a crystal may be 

 cut specifically different from each other, which require to be 

 distinctly understood. 



In the first place, let us suppose a prismatic crystal to be 

 placed with its axis in a vertical position, and a portion to be 

 cut off from the base by a plane surface at right angles to the 

 axis, and sufficient to form a wedge of 20 degrees, by giving it 

 a second surface duly inclined to the former. For distinction, 

 this may be called the horizontal wedge. 



Next, let the crystal be bisected vertically by a plane passing 

 through two opposite edges of the prism, in order to make two 

 other wedges which are to be cut in different directions from the 

 two portions, and to have each the same angle of 20 degrees. 



Let one of the halves thus obtained be slit in a plane which 

 meets the surface of bisection in one of the edges of the original 

 prism, and consequently in a line parallel to the axis. The 

 wedge thus formed may be called a lateral wedge. 



Let the remaining half be cut by another plane not vertical, 

 but inclined to the vertical plane at an angle of 20", and meeting 

 it in a line parallel to the base, or at right angles to the axis. 

 This may be called a vertical wedge. 



We have thus three wedges cut in different directions at right 

 angles to each other, and, accordingly, having their axes of cry- 

 stallization differently placed in each. 



In the first, or horizontal wedge, the axis is at right angles to 

 the first surface. In the second, or lateral wedge, the axis is 

 parallel in the first surface, and parallel to its acute edge. In 

 the third, or vertical wedge, the axis is also in the first surface, 

 but it is at right angles to the acute edge. 



An ol)ject seen through the first wedge in the direction of the 

 axis, does not appear double ; but, since rays transmkted through 

 the second or third pass at right angles to the axis, both of these 

 wedges give two images of any object seen through them. 



There are obviously three modes in which these wedges may 

 be combined in pairs, by placing two of them together with their 

 acute edges in opposite directions. The first pair 

 may be represented by L H ; the second by V H ; L^ 

 the third by V L. In the two first cases the separation of the 

 images will be the same : since the angles of all the wedges are 

 supposed to be made c(iual, the compound medium will be com- 

 prised under parallel surfaces, so that a ray ordinarily refracted 

 by both emerges in its original direction ; but since the extra- 

 ordinary ray is made to deviate about 17 minutes from the or- 

 dinary 



