256 USE OP THE MICROSCOPE. 



measured by the reflective goniometer. Another source of 

 error sometimes arises from not observing that the planes 

 measured are those of macled or aggregated crystals, and thus 

 furnish angles which would not exist in a distinct crystal." 



Mr. Ross has also contrived a very excellent goniometer ; 

 it is constructed somewhat on the same principle as the 

 micrometer eye-piece, shown at fig. 134, page 212, by placing 

 a cobweb in the lower part of the outer tube in the focus of 

 the eye-glass, instead of a divided scale, and having the upper 

 rim of the same tube divided into 360". By turning either 

 the crystal or the cobweb, so that one of the sides of the 

 former may lie in a line parallel with the cobweb, and having 

 set the index at zero, or observing the degree it points to, 

 and then bringing the cobweb in a line parallel with another 

 side of the crystal, the number of the degrees passed over 

 will give the angle required. 



