MEASUREMENT OF CRYSTALS. 61 



Prism will form a great number of series of particular rhom- 

 bic prisms, varying from each other in their relative heights, and 

 in the angle at which the lateral planes incline to each other. Those 

 of the system of crystallization belonging to the Rhomboid will com- 

 prehend a great number of series, according as the Rhomboids differ 

 in the angles at which their planes incline, respectively, to each oth- 

 er, &c. 



. 57. METHODS FOR ASCERTAINING THE ANGLES or 

 CRYSTALS. 



From what has preceded relating to the differences among 

 the forms of crystals, and the systems of crystallization, it 

 is apparent, that in order to become acquainted with crys* 

 tals, it is requisite'we should possess the means of measuring 

 their angles with precision. This is effected by the aid of 

 instruments called Goniometers, which are of two kinds : 

 viz. the Common Goniometer, and the Reflective Goni- 

 ometer. 



1. Common G-oniometer. 







Figures 1, 2 and 3, give a representation of this instrument. In 

 Fig. 1, the half circle or reporter is attached to the arms; in 

 the other figures, the arms and the semicircle are separate. The 

 first of these was invented by Carangcol about fifly years ago. 

 The half circle s t r, made of brass or silver, is graduated into 180 

 degrees, each degiee being marked on the instrument by a short 

 line extending from the outer rim to the circle which is next 

 within it, and a mark for every five degrees extending still farther 

 towards the centre, until it cuts a second circle, as may be seen by 

 inspection of the figure. A plate of steel or brass extends a little 

 by the centre from r towards s, in order to support the axis c, about 

 which moves the moveable arm df. This arm may be lengthened 

 or shortened in the direction c d, by means of the slit I m. Two 

 similar slits, g h, i A:, in the fixed arm a 6, allow of its being moved 

 forwards or backwards upon the two fixed points c and e. These 

 two arms are made of steel. It is obvious that by this provision, it 



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