1070 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



the pinion wire, the object being to allow of the raising or lowering 

 of the body of the microscope for focussing. The analyser and the 

 polariser may thus be rotated synchronously without disconnecting 

 their toothed wheels. The polariser, in the latest form of the 

 instrument, is mounted on a crank arm, so that, if not required, it 

 may be thrown out of the axis of the stand. Now, in the microscopes 

 usually constructed for petrological work the rotation of a small 

 crystal on the stage between the polarising and the analysing prisms 

 is liable to put it out of position in regard to the cross-threads in 

 the eyepiece, as the centring of the objective is scarcely ever so 

 perfect as not to produce some displacement ; and, if the centring 

 be adjusted so as to be perfect for one objective, it is likely to be 

 faulty for another. (By a small crystal is meant a crystal under the 

 ToVoth of an inch in diameter, and of such thickness as one finds at 

 the edges of petrological sections.) Hence, by the arrangement 

 described above, centring is dispensed with, and the object is made to 

 rotate between the two prisms of the polarising apparatus without 

 changing its position beneath the objective. To a petrol ogist who 

 is accustomed to a rotating stage and fixed cross-wires, a familial- 

 section appears strange when first looked at on a fixed stage with 

 movable cross- wires, -but after a few hours' work with the instrument 

 the feeling of strangeness passes and that of the solid advantage of 

 a perfect centring remains. 



On the polariser tube, above the toothed wheel and below the 

 stage, is fitted a goniometer, D, which, in combination with crossed 

 lines in the eyepiece, will permit of the measurement of the angles of 

 crystals without necessitating the shifting of the object when once 

 adjusted in the field. is a set screw by which the polarising 

 apparatus and goniometer may be fixed in any desired position. 

 Both the analysing and polarising prisms are divided to every 45, a 

 spring catch marking the extinction point. The opening between 

 the upper lens of the eyepiece and the analysing prism B ; (fig. 803) 

 is for the purpose of placing such plates as the ^-undulation plate K 

 in position. 



The great value of the instrument is in the facility with which 

 studies in convergent light can be performed. G is a slide fitted 

 with a double convex lens which may be used for showing the 

 optical figures of crystals, and H is a similar slide carrying a lens 

 and a diaphragm of small aperture used for showing optical pictures 

 in minute crystals. The polariser is fitted with two convergent lenses, 

 which work in conjunction with the lens A. on the slide of the stage, 

 when great convergence is required. This slide may be pushed in 

 without disturbing the object upon the stage. The achromatic con- 

 denser, A, shown at the foot of the figure, also works in conjunction 

 with the sliding lens, A, when the highest angular aperture is required. 1 



1 In the latest made instruments a new achromatic convergent system is intro- 

 duced over the polariser. It gives a N.A.. of TOO, and an aplanatic cone 0'92. When 

 used as an immersion condenser, these are increased respectively to T12 and 1'05. 

 It is fitted with an iris diaphragm placed above the polarising prism. A milled 

 collar actuates the focussing of the lower portion of the condenser. The fine adjust- 

 ment is the differential- screw form, which is sufficiently delicate and accurate to 

 determine the refractive index of minerals by the difference between the focus -taken 



