700 SUMMARY OF CURRENT RESEARCHES RELATING TO 



modification) on the Continent, and was originally devised by M. 

 Nacliet, Senr. The rotating movement has the disadvantage, common 

 to the older forms of movable stage, of being acted upon by the 

 rectangular movements, so that the centering in the optic axis is 

 disturbed whenever these movements are used, their utility being 

 thereby seriously reduced. 



The Microscopes now issued by the firm of Chevallier are on a 

 more modern type : their large stand is shown in fig. 125. 



Hirschwald's Microscope-Goniometer.* — J. Hirschwald has de- 

 vised a Microscope-goniometer for measuring the angles of crystals 

 not having reflecting surfaces, the principle of which consists in 

 employing the sensibility of a Microscope in the accurate focusing 

 of a plane surface. The instrument (fig. 126) consists essentially of 

 three parts — (1) a Wollaston goniometer, (2) a Microscope, and (3) a 

 telescope. 



The goniometer is firmly attached to a horizontal base-plate C. 

 The circle M is divided into half-degrees, and by means of a ver- 

 nier N reads to single minutes. The lens L allows of a still closer 

 estimation. By the milled head 0^ the object can be turned without 

 the circle, the movement of the latter simultaneously with the object 

 being effected by turning the larger milled head O^. By screwing 

 down P the turning of 0^ is prevented, and the circle with the object 

 can then only be moved by the screw Q, which presses against the 

 end of the lever J attached to the axis, giving a very slow movement 

 to the circle. 



The crystal is attached to the holder E, which, besides rotating 

 on the goniometer axis, allows of four other motions, viz. two rect- 

 angular movements in a plane at right angles to the axis, and two 

 similar movements, but in segments of a sphere. 



The Microscope rests on a double slicle D E, by means of which 

 it can be moved either parallel or at right angles to the axis of the 

 goniometer, so that the entire surface of a crystal can be examined. 

 The slide E has an index mark, which indicates the extent of move- 

 ment upon a scale G in half-millimetres. The slide D can be fixed 

 to the base-plate by clamping the screw H. The micrometer-screw 

 F of the Microscope has a pitch of 0-4 mm., and is graduated so 

 that the raising or depression of the Microscope can be read to 

 0"004 mm. The eye-piece has cross-threads, one parallel and the 

 other at right angles to the axis of the goniometer-circle. The 

 former will lie in the vertical plane of the axis when the Microscope 

 is adjusted so that the index mark is at the zero of the scale G. The 

 Microscope with the three lenses of the dividing objective has a mag- 

 nifying power of 500 times, a focal distance of 0"76 mm., and a 

 sensitiveness of focus of 0-0015 mm. Without the lower objective 

 the figures are 350, 1 • 2 mm., and • 004 mm. respectively ; and on 



* Nenes Jahrb. f. Mineral. Geol. u. Palaeontologie, 1879, pp. 301 (1 pi.) and 

 539 ; 1880, p. 136. 



