184 HOW TO WORK 



plate that carries the body, prism box P, object-glass, and fine ad- 

 justment A : to the side of the base is fixed a square bar G, that 

 carries the principal stage with its coarse adjustment, and the secon- 

 dary stage into which fits the diaphragm, polarising bundle B, selenite 

 plates, &c. A tube screws into the top of bar G, on which slides 

 the mirror. The body slides into a socket attached to the prism box. 

 Within the draw tube are fittings to receive glass tubes for examining 

 with a Leeson's goniometer and minute stop, the amount of rotation 

 in liquids that exhibit circular polarisation. 



The prism P, that reflects the image of the object up the axis of 

 the body at a convenient angle for observation, is contained in a 

 solid brass box, on the upper surface of which are screwed the tubes 

 and fine adjustment A that carry the object-glass, and one side is 

 removeable to allow of the prism being readily taken out and 

 cleaned. 



A semicircular arm works up and down the upright bar G, by 

 means of a rack and pinion R, and supports the circular stage S, 

 which for ordinary work is kept in a horizontal position by means 

 of the clamp nut N. The stage has a projecting ring, within which 

 a graduated plate rotates when optical examinations have to be 

 made : but which is ordinarily fitted with a plain metal plate that 

 rises flush with the top of the axes of the stage. In this instrument 

 the object has to be placed with the glass cover downwards. 



A shore body replaces the ordinary one for optical examinations ; 

 this is fitted with a tourmaline T, and a cell for a plate of calc spar 

 C, when the instrument is to be used as a modification of Professor 

 Kobell's stauroscope for determining crystal systems ; and two lenses 

 L L with a Jackson's micrometer M, when the instrument is required 

 for the determination of the optic axis on the principle of Soleil's 

 instrument. 



For such investigations consult the Mineralogical Works of 

 Dufrenoy, Delafosse, Descloizeaux, and Grailich. 



At the present date Mr. Highley arranges all apparatus required 

 far investigating mineralogical or crystallographic optics, including 

 Norremberg's recent system of lenses, so as to be adaptable to most 

 of the ordinary forms of microscope, and prefers using a cheap form 

 of microscope with a prism having a single reflecting surface instead 

 of two, as in the instrument figured in pi. XLIII, for observing crys- 

 tallisations or corrosive or fuming objects. 



291. Of Preparing Fossils for Microscopical Examination. Dif- 

 ferent methods of preparation are required in examining the various 

 fossils. Many kinds of fossil bone and some forms of teeth may be 

 prepared according to the directions given in p. 82. In cases in 



