CONTRIBUTIONS TO MICRO-MINERALOGY. 283 



A shorter body for other optical examinations replaces the 

 ordinary one ; this is fitted with a Tourmaline T, a cell for a 

 plate of calc spar C, [cut at rioht angles to the principal axis 

 of the Rhombohedron or Hexagonal Prism,] when the instru- 

 ment is to be used as a modification of Professor Kobell's Stau- 

 roscope for determining Crystal-Systems ; and two lenses L L, 

 with a Jackson's Micrometer M, at the point of their foci, when 

 required for the determination of the optic axes on the principle 

 of Soleil's Instrument. 



The Prism i», is Contained in a solid brass box, on the upper 

 surface of which is screwed the tubes that carry the object glass ; 

 and one side is removable to allow of the prism being readily 

 taken out and cleaned. 



The prism itself is six-sided, and has four polished faces with 

 angles of such dimensions, that a ray of light reflected down the 

 axis of the object-glass suffers two internal total reflections, the 

 second being axial to the body of the Microscope. The angles 

 and quality of the glass are ])oints of the greatest importance in 

 the construction of this instrument, as also are the proper adap- 

 tations of the object-glass and body to the line of reflection. 



Angles. J. L. Smith. 

 Upper obtuse = 145^. 



,, acute = 55°. 

 Lower obtuse = 107|^^. 



„ acute = 52^°. 



The Fine Adjustment cousists of a tubc scrcwed into the 

 top of the prism-box at right angles to its surface ; over this, 

 tio-htly, but smoothly, slides another tube on which the object- 

 glasses are screwed. This is kept up to its work by a spring of 

 coiled wire, on which it rests ; at right angles to its base an 

 arm projects, through which the fine-wormed screw of the 

 milled-head adjuster A works. The spindle of the adjuster A, 

 rotates in a socket projecting from the prism-box. 



The i^tage differs materially in its construction from that of 

 Professor Smith's or Nachet's, and allows a far wider scope for 

 pursuing the physical examination of mineral bodies. A stout 

 semi-circular arm works up and down the upright bar G, by 

 means of a rack and pinion R. This supports a circular stage 

 S, fio«. 4 and 5 ; two axes at the circumference of the stage 

 pass through the extremities of the arm ; the stage being 

 kept in a horizontal position by means of the nut ??, which 

 passes through the arm into the stage ; the nuts, N, screwing 

 on to the axes, clamp the stage firmly to the arm. The stage 

 has a projecting ring, within which a graduated plate rotates 

 when certain optical examinations have to be made, but which 



