CONTRIBUTIONS TO MICRO-MINERALOGY. 285 



point 1, fig. 4, rises from the stage arm as the starting-point for 

 the readings. Thus, in measuring the optic axes of a crystal, 

 the stage is inclined till one optic axe is properly cut by the 

 micrometic lines M, in the short body ; the graduated circle 

 rotated till zero stands opposite the index-point I, and is there 

 clamped ; the stage is again inclined till the second axe cuts 

 the micrometic lines, and as the graduated circle has passed on 

 with the stage, the readings are taken from the fixed index- 

 point I. This circle is graduated from 1 to 180 on each half, 

 so that the reading may be taken on whichever side the stage 

 may be inclined. Its use as a Wollaston goniometer, and other 

 operations for which this is adapted, w-ill be given under the 

 proper heads of these Coxtkibutioxs. The graduated circle 

 also fits over an eye-piece containing a double-image prism, and 

 thus constitutes a Leeson's goniometer. The manipulation 

 and details of construction will be given under the head of 

 " Goniometry." It will thus be seen that one graduated circle 

 economically does the work of three by this arrangement. 



The secoudaiT .sitngc requires no special description, as the 

 ordinary accessory instruments are mounted so as to slip on a 

 ring that rises from its surface. I find a reflecting bundle of 

 thin glass the best Polarizer for this form of Microscope ; by 

 removing the back and using the plain mirror, it may also be 

 used as a Refracting Polarizer. An electro-magnet arranged 

 so that its poles, which terminate in small sliding cones, may be 

 brought into the field of view without intercepting much of the 

 light reflected from the mirror, is mounted on a fitting, so that 

 wlien required it slips on the square bar G, between the princi- 

 pal and secondary stage. 



It will be readily seen that this form of microscope possesses 

 great advantages for chemical and mineralogical investigations, 

 as the stage is in a far better position for the eye to watch the 

 manipulations than in the ordinary instruments ; and the object- 

 glass being under the object cannot be dimmed or attacked by 

 the vapours arising from the liquids under examination. In 

 fact, Professor Bailey uses hydro-fluoric acid to determine 

 whether markings on a siliceous body are to be regarded as 

 elevations or dej)ressions, as those parts that are elevated will 

 be last seen under the dissolving action of the acid. Professor 

 Riddell, in a note to Professor Smith, states, that after tw^elve 

 months' trial he will not willingly return to the habitual use of 

 any known form of microscope, " especially with high powers." 



Moreover, as the plate that carries the l)ody and object-glass 

 rotates on the base, when the parts are properly centred, this 

 may be used as a Demonstrating Microscope, as the body can 



