MEASUREMENT OF CRYSTALS. 



grees. The arms are so applied to the scale, that the ceilter 

 of motion is exactly at the extremity of the middle' /line, 

 marked 90 ; and the leg crossing the scale (or diat edge of it 

 in the line of the center of motion) will then indicaj^ by its 

 position over the graduated margin, the angle desii 



In making such measurements it is important to/remember 

 that — 



1. An angle A O D (figure 87) and A C, together, 

 equal 180° ; so that if A O C be measured, A O D is ascer- 

 tained by subtracting A O C from 180°. / 



2. In a rhomb or rhomboid, b a b and /t b «, to- j^ 

 gether, equal 180°; and one may be ascertained 

 by subtracting the other from 180°. /f an obtuse 

 angle of a rhombic prism has been measured and 

 found to be 110°, and the acute angle on measurement is as 

 certained to be 60°, the student sdiould add the two together 

 to find whether the sum is 18QP ; for if not, there is some 

 error in the measurement, and/it should be repeated. 110 

 added to 60° makes 170°, showing in this case an erroi 

 of 10°. 



3. In any polygon, the sfim of the angles is equal to twice 

 as many right angles as there are sides less two. Let the 

 number of sides, for example, be & : 6 less two is 4 ; and 

 the angles together equal twice 4, (or 8,) right angles, which 

 is equivalent to 8 X 90° =720°. If we have a prism of six 

 sides, and wish to ascertain the angles between these sides 

 the angles should be measured successively, and the whol< 

 added together to/ascertain whether the measurements ar( 

 correct. If the anm is 720°, there is good reason to confide 

 in them. Crystals are at times a little irregular ; and this: 

 should be looked tg, as part of the apparent error may at 

 times be thus accounted for. This general principle and the 



/ . . 



What thre^ points must be observed in making measurements 1 



* Another mode for approximate results consists in holding the crys- 

 tal with the two faces (whose inclination is to be measured) in an 

 exactly vertical pesition over a piece of paper : then place a small rule 

 parallel,' as near as the eye can judge, to one face, and draw a line ; next 

 do the same for the other face. The angle between the two lines, 

 measured either by an arc or the ivory rule just mentioned, is the. 

 desired inclination. With practice, much skill may be acquired in 

 such trials. They may be made with micro? ,opic crystals under 

 microscope. 



