THE REQUIEEMENTS OF THE PETROLOGICAL 

 MICROSCOPE. 



By Dr. J. W. Evans, F.R.S. 



The Petrological microscope is constructed to serve two purposes. 

 It is employed, in the first place, as an ordinary microscope, to 

 observe the form and structure of the smaller features of rocks ; and it 

 is also used as an optical instrument for studying the action of minute 

 crystals on light with a view to their identification. The latter 

 function requires special features of greater or less complexity. The 

 exact nature of these arrangements depends, however, to some extent 

 on whether the material is examined in the form of a thin section 

 of a rock, or in minute grains or fragments. 



In all petrological microscopes provision is made for the examina- 

 tion of the object between crossed nicols, and for the rotation of these 

 or of the stage or of both alternatively. The advantage of a rotating 

 stage and stationary nicols is so great from the point of view of sim- 

 plicity of construction, that it is always adopted in the cheaper in- 

 struments, and it is quite satisfactory in all eases where the work is 

 confined to thin sections and methods involving certain special acces- 

 sories or arrangements are not required to be employed. 



On the other hand, for the examination of grains mounted in oil 

 or other highly refracting medium, the use of a stationary stage and 

 rotating nicols is practically a necessity, if high powers are to be 

 employed, unless the Nachet device is adopted, by which the objective 

 is attached to the stage and rotates with it. Rotating nicols are also 

 necessary for the more complex optical methods, especially those that 

 require an axis of rotation at right angles to the optical axis of the 

 microscope, as when the optical characters of crystals are studied by 

 means of the theodolite or " universal " stage. It deserves considera- 

 tion whether, when rotating nicols are employed, a rigid connection 

 T^etween them should not be substituted for the gearing employed 

 by Dick, even although the former is open to the objection that a 

 rotation through a complete circle is not possible. This course has 

 been occasionally followed. 



Where crushed material or small grains are examined in oil or 

 micro-chemical tests are applied, the microscope should be protected 

 by a shallow glass bath with a plane floor, large enough to hold the 

 glass slip. 



There should be a '' mechanical stage " providing for the move 

 ment of the object in two directions at right angles to each other and 

 to the optic axis of the microscope, so that the position of the object 

 may be varied while its orientation remains unaltered. These move- 

 ments and the fine adjustment should be accurately graduated. 



Arrangements should also be made by which a nicol may be placed 

 in a position above the eye-piece. At the same time a slot should 

 be provided at the focus of the eye-piece, so that accessories, such 

 as quartz wedges, may be inserted in focus. The upper nicol or 

 analyser, wherever placed, should be capable of rotation, either simul- 



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