48 STRUCTURE OF MINERALS. 



the required angle. To insure accuracy in this respect, hold 

 the instrument and crystal between the eye and the light, 

 and observe that no light passes between the arm and the 

 applied faces of the crystal. The arms may then be secured 

 in position by tightening the screw at o ; the angle will then 

 be measured by the distance on the arc from k to the lefi 

 or outer edge of the arm c d, this edge being in the line oi 

 o, the center of motion. As the instrument stands in the 

 figure, it reads 45°. The arms have slits at g h, n p, b} 

 which they may be shortened so as to make them more con- 

 venient for measuring small crystals. 



In some instruments of this kind the arc is detached from 

 the arms. When this is the case, after the measurement is 

 made and the screw at o tightened, the arc (which has the 

 shape of a f b in the annexed figure, except that from a to b 

 is a solid bar) is adjusted to the upper edge of one of the 

 arms, bringing the mark at o, the center, exactly to the center of 

 divergence of the arms. The angle is then readoffas before. 



With a little ingenuity the student may construct a goni- 

 ometer for himself that will answer a good purpose. A semi- 

 circle maybe described on mica or a glazed card, of the 

 shape in figure 88 : it should then be divided into halves at 

 /*, and again each half subdivided into nine equal parts. 

 Each of these parts measures 10 degrees ; and if they are 

 next divided into ten equal parts, each of these small divisions 

 will be degrees. The semi-circle may then be cut out, and 

 is ready for use. The arms might also be made of stiff card 

 for temporary use ; but mica, bone or metal is better. The 

 arms should have the edges straight and accurately parallel, 

 and be pivoted together. The instrument may be used like 

 that last described, and will give approximate results, suffi- 

 ciently near for distinguishing most minerals. The ivory 

 rule accompanying boxes of mathematical instruments, having 

 upon it a scale of sines for measuring angles, will answer 

 an excellent purpose, and is as con- 

 venient as the arc. The annexed 

 figure will illustrate the mode oi\ 

 using it. The scale is graduated 

 along the margin, the middle point \ 

 marking 90°, and the divisions 

 either side 10 degrees (as in the figure) and also single de« 



How is it used when the arms are detached ? How may a temporary 

 j>oniometei be made 1 How may a scale of sines be used 1 



