210 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



line of which stands out against the sky, and for the lower one the edge 

 of a table or of a sheet of paper placed on it. 



That done, the goniometer is placed in such a position that the 

 limb is exactly vertical (a level and feet with screws enable this 

 result to be obtained), and at the same time in a direction perpen- 

 dicular to the sights chosen. Afterwards the crystal is fixed on the 

 plate of the instrument by means of wax ; and it is necessary to 

 place it in such a way that the edge of the angle to be measured be 

 itself perpendicular to the limb or parallel to the axis of rotation. 



FIG. 147. Wollaston's reflection goniometer. 



To secure this, images of the two sights obtained by reflection on 

 both faces are used, images which, for each of the faces, ought to be 

 parallel between themselves. 



When these preliminary arrangements are once taken, the zero of 

 the limb is placed in coincidence with the zero of the vernier. Then 

 turning the crystal by means of the milled head A, the image of the 

 upper sight as seen in the first face is brought into coincidence with 

 the lower sight, seen directly. Then the limb is turned, and con- 

 sequently the crystal, until the same coincidence is obtained, this time 



