3i6 SCIENTIFIC APPARATUS. 



neither of these will, in that case, coincide with the trace of 

 the plane of primitive polarisation. By the rotation of the stage 

 the crystal may next be brought into a position of maximum 

 obscuration : and after now turning the rotating wire of the 

 ye-piece into a position of optical coincidence or close paral- 

 lelism to the edge or direction assumed as known in the crystal, 

 we may measure the angle through which the crystal has been 

 moved to effect this result. That angle gives the inclination of 

 the principal sections of the section of the crystal to the known 

 crystallographic direction. 



The applications of this method are rather more limited than is 

 the case with the stauroscope ; but, practically, its chief use is 

 to determine the crystallographic system to which the mineral 

 belongs in cases where that mineral is generally one among a few, 

 the crystallographic and optical characters of which are perfectly 

 familiar to the scientific petrologist.* 



The objects sought to be attained under class i and class 3 

 are fulfilled by the improved polarising microscopes of Norren- 

 berg and of Des Cloizeaux. In these the requirement is to cause a 

 beam of plane polarised light to traverse the portion of the crystal 

 to be examined, which too often is very minute, so as to give the 

 greatest possible difference of path for the ordinary and extra- 

 ordinary rays within the section, and to obtain the largest possible 

 field. In the French instrument the light is polarised by reflec- 

 tion ; in that of Norrenberg by being made to converge near the 

 middle of a Nicol prism. In the latter the crystal section to be 

 examined is placed on a stage, above and below which a series 

 of lenses, symmetrical in their arrangement as regards the section 

 of the crystal, cause the plane-polarised light to traverse the 



* It was by these means that the writer was enabled, by measurements of 

 the inclinations of edges of sections, and recognising the directions relatively 

 to them of cleavage planes, to determine the presence in meteorites of a 

 prismatic mineral other than Olivine, and to which he assigned the characters 

 of Enstatite (or Bronzite) before the presence of the latter minerals had been 

 confirmed by chemical analysis. 



