252 



USE OF THE MICEOSCOPE. 



remove many of the difficulties incident to the instruments 

 heretofore in use, is represented in figs. 159 to 167. Amongst 

 the peculiar advantages of this instrument may be enumerated 

 its capability of measuring opaque and imperfect crystals, also 

 microscopic crystals, and crystals in the interior of transparent 

 media. It is equally apphcable to the largest crystals, and 

 will measure angles without removing the crystal from a 

 specimen, provided only the whole is placed on a suitable 

 adjusting stage. The instrument depends on the apphcation 

 of a doubly refracting prism, either of Iceland spar or of 

 quartz, of such a thickness as will only partially separate the 

 two images of the angle which it is proposed to measure. 



" Premising that the goniometer may be either mounted as 

 a separate instrument or attached to a microscope, it is pror 

 posed to describe it as when fitted to a body of the improved 

 compound microscopes in common use. The same letters 

 will be used for the corresponding parts in figs. 159 to 164, 

 which represent the various parts of the goniometer fitted up 

 as the eye -piece of a microscope, together with those of the 

 adjusting stage to support the crystal which is intended to fix 

 upon the traversing stage of the microscope. 



"Fig. 159 is a perspective view of the goniometer, and fig. 

 160 is a section of the same. At a is an achromatic prism of 



Fig. 159 



double refracting Iceland spar — a Kochon's prism of quartz 

 may be substituted; 6 is a brass tube containing the prism, 

 with a round aperture forming the cap of the eye-piece, and 

 sliding stiffly on the tube c attached to the arm d carrying the 

 vernier of the graduated circle /*. At /is an achromatic eye- 



