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X. — On a Pefrographical Microscope* 

 By A. Nachet, F.R.M.S. 



(Read 10th March, 1880.) 



A Microscope intended for petrographic investigations should 

 have a quality somewhat difficult to obtain, that is, the perfect 

 centering of the stage beneath the objective so that a crystal 

 examined by polarized light is always in contact with the crossed 

 threads placed at the focus of the eye-piece. 



If the stage is made to revolve under the objective the latter 

 must be perfectly centered, that is to say, it must be placed in 

 absolute coincidence with the axis of rotation of the stage. Several 

 methods have been proposed to accomplish this, but they have 

 all a grave inconvenience, viz. that when the objectives are 

 changed it is necessary again to find the axis of rotation, to which 

 the axis of the new objective no longer corresponds, thus causing 

 considerable loss of time. 



If, however, the objective is turned with the object there is no 

 longer any displacement of the image. To accomplish this it is 

 only necessary to arrange the optical part so that it is divided 

 into two sections, the objective forming" part of the stage and 

 turning with it, and the eye-piece remaining fixed with the crossed 

 threads and the analyzer. 



A Microscope designed on this principle is shown in Fig. 15. 

 The column A carrying the fine and coarse movements, only 

 supports in reality the objective, and the whole turns with the 

 stage and the object, the upper tube containing the eye-piece and 

 the analyzer remaining immovable. The latter tube slides into a 

 collar at the end of the arm fixed to the column B, so that it is 

 not affected by the rotation of the body nor by the focal adjust- 

 ment which acts only on the objective. 



The advantage of this instrument is shown by tlie fact 

 that the greater the amplification the more precise the centering 

 is, contrary to what is the case when the stage alone turns beneath 

 the objective. In fact the sole cause of deviation which can 

 exist in the new system arises from a small alteration of coinci- 

 dence between the axis of the cye-pioce and objective, but this 

 want of coincidence is almost imperceptible. 



In examhiing rocks it is necessary to change the objectives 

 very frequently — a revolving nose-piece can scarcely be employed 

 in consequence of the flexion which exists in those for three or 

 four objwtives, notwithstanding every care in construction. 1 



* Tin- ori'^iiml |)!ii)or is in I'Vuncli. 



Q 2 



