OF THE GONIOMETER, 27 



Fig. 54. 



When this instrument is applied to the planes of a 

 crystal, the points d and/, fig. 52, should be previously 

 brought sufficiently near together for the edges e?e, 

 efy to form a more acute angle than that about to be 

 measured. The edges being then gently pressed upon 

 the crystal, the points d, and /, will be gradually 

 separated, until the edges coincide so accurately with 

 the planes, that no light can be perceived between 

 them. 



The common goniometer is however incapable of 

 affording very precise results, owing to the occasional 

 imperfection of the planes of crystals, their frequent 

 minuteness, and the difficulty of applying the instru- 

 ment with the requisite degree of precision. 



The more perfect instrument, and one of the high- 

 est value to Crystallography, is the reflective gonio- 

 meter invented by Dr. Wollaston, which will give the 

 inclination of planes whose area is less than To^Vso 

 of an inch, to a minute of a degree. 



This instrument has been less resorted to, than 

 might, from its importance to the science, have been 

 expected, owing* perhaps to an opinion of its use 

 being attended with some difficulty. But the ohserv- 

 ance of a few simple rules will render its application 

 easy. 



D 2 



