98 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



scope attached to the rim of the glass face, movable so as 

 to be easily placed on any part of it, for the purpose of 

 reading off the dip with greater accuracy. 



To use the instrument, set the graduated rim in the plane 

 of the magnetic meridian by means of a common compass, 

 levelling it by means of the screws of the stand. 



Price, $40.00 to $50.00. 



The Goniometer. (Fig. 

 110.) An instrument for 

 measuring ano-les, and more 



3 & " 



particularly the angles form- 

 ed by the faces of crystals. 

 The common goniometer 

 consists of a pair of steel 

 blades moving on a centre, 

 as shown in the cut ; the 

 edges, a a, are carefully ad- 

 justed to the faces of the 

 crystal, whose inclination to each other it is required to as- 

 certain ; and then the instrument being applied to the 

 divided semicircle, the angle contained is at once read off. 

 An approximate measurement, within one or two degrees, 

 can be easily obtained by this instrument, provided the planes 

 of the crystal be tolerably perfect, and large enough for the 

 purpose. Price, $3.00 to $5.00. 



Fig. in. Wollastoii's Reflecting Goniometer. 



(Fig. 111.) The reflecting goniometer 

 is a very superior instrument, its indica- 

 tions being correct within the fraction of 

 a degree ; it is applicable also to the 

 measurement of the angles of crystals 

 of very small size, the only conditions 

 required, being, that their planes be 

 smooth and brilliant. It consists of a 

 brass circle, A, graduated on the edge, 

 and furnished with a vernier, E, by which 

 the divisions may be read accurately to minutes. The circle 

 moves in a vertical plane, and is supported on a stand. The 

 axis of the circle is a hollow tube, within which is a smaller 

 axis, fitting so tightly that when turned round it carries the 

 other axis, and consequently the wheel, along with it, unless 



