348 
SUMMARY OF CURRENT RESEARCHES. 
an iron tripod foot after the English style, and the illuminating appa- 
ratus is worked, not by means of a screw, but by freehand motion. The 
iris diaphragm is applied by a side movement, and consequently affords 
a better control of its opening. A movable and rotatory stage can be 
fitted if desired. 
Brugnatelli’s Large-size Mineralogical and Petrological Micro- 
scope.* — The stand (fig. 43) is 40 cm. in height, is mounted on an iron 
horse-shoe foot, and possesses perfect stability at all inclinations. The 
illuminating arrangement can be raised or depressed at will by means of 
rackwork. The polariser and condenser are centered by two screws and a 
spring. A large Nicol’s prism is mounted in a rotatory tube, completely 
independent of all other movements ; it can be set in all desired orien- 
tations, and can be easily removed for cleaning. Immediately below the 
Nicol is an iris diaphragm, whose movement, regulated by a button, is 
completely independent of that of the polarising prism, in such a way 
that the motion of the diaphragm has no effect on the orientation of the 
prism. A division into degrees and an index gives this orientation ; 
another division allows the opening of the diaphragm to be read in 
millimetres. This new arrangement is useful for the determination of 
the indices of refraction after Viola’s method. The condenser is com- 
posed of three lenses, of which the largest, with very long focus, is united 
to the mounting of the Nicol, and serves also for observations with 
parallel light. The means by which parallel light may be exchanged 
for very convergent illumination is imitated from the Fuess Microscope, 
but simplified in the sense that the condenser can be easily unshipped 
without the aid of a special key. The two superior lenses of the con- 
denser, forming a system with large aperture angle, are mounted on a 
movable arm fixed under the stage. The arrangement for inserting this 
and taking it away is the same as in the Fuess Microscopes. 
The circular stage is 120 mm. in diameter, and bears on its periphery 
a scale in degrees on argentan (a metal which keeps its white colour). 
It moves on a conical bronze guide with great precision. Two verniers 
read to 10'. On the rotatory stage is fixed a micrometric car of very 
careful construction whose orthogonal displacements are regulated by 
two micrometric screws each provided with a drum giving, the one, 
hundredths of a millimetre, the other, 0*04 mm.; two divisions in 
millimetres give the entire turns. The maximum movement of the car 
is 20 mm. in each direction. The object-carrier is applied to the car by 
means of an easily removable clip. 
The upper part carrying the tube has been heightened by 4 cm. by 
means of a massive cylindrical column ; this arrangement, coupled with a 
more extended draw-out of the tube by means of a rack, allows the placing, 
between the stage and the objective, of accessories of a certain height, 
such as the stages of Klein, Fedorow, &c. The tube is composed of 
two parts jointed one within the other. The lower extremity bears a 
revolver for three objectives, centerable on the rotation axis of the stage. 
By means of always screwing the same objective on the same arm of the 
revolver, a perfect centering is obtained. The centering ring of the re- 
volver is pierced by an opening of rectangular section orientated at 45°, 
* Bull. Soc. Vaud. Sci. Nat., xxxiii. (1897) pp. 228-30 (1 pi.). 
