

CRYSTALLOGRAPHY. 11 
The reflecting goaiometer affords a more accurate method of- 
measuring crystals that have lustre, and may be used with those of 
minute size. The principle on which this instrument is constructed 
will be understood from the annexed figure, representing a crystal, 
whose angle @0 cis required. The eye, look- 
ing at the face of the crystal ) ¢, observes a 
reflected image of m, in the direction Pn. On 
revolving the crystal till a > has the position of 
bc, the same image will be seen again in the 
same direction P 7. As the crystal is turned, 
in this revolution, till @ 6 @ has the present 
position of } c, the angle d 4) c measures the number of degrees through 
which it is revolved. But d@ 5c subtracted from 180° equals the angle 
of the ecrystala bc. The crystal is therefore passed, in its revolution, 
through a number of degrees equal to the supplement of the required 
angle. 
This angle, in the reflecting goniometer of Wollaston, is measured 
by attaching the crystal to a graduated circle which revolves with it, 
one form of which is here represented. 




















































































































































































































Cis the graduated circle. The wheel, m, is attached to the main 
axis, and moves the graduated circle together with the adjusted crystal. 
