350 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and intended to receive tlie feather edges of gypsum and quartz. This 
opening can be closed at will by a movable ring. Immediately above 
this arrangement is a window in the exterior tube, in which slides the 
analysing Glan-Thompson prism combined with a long focus lens which 
renders invariable the focal distance of the optical system used with or 
without the analyser. A new arrangement allows the orientation, by a 
rotation of 90°, of the analyser ; the orientation of the principal section 
boing indicated by an index and a divided dial. All the upper part can 
be raised or lowered by means of a rack and pinion ; the micrometric 
movement is done by means of a screw whose head is divided into *005 
mm. 
In order to use the instrument as a focimetcr, a division in milli- 
metres is placed on the lateral part of the rackwork piece, and a vernier 
below the pinion. 
The eye-piece tube is also worked by a rack and pinion, and can be 
extended about 40 mm. A millimetre scale reads the extension. 
In the lower part of the eye-piece tube is an iris-diaphragm opened 
and shut by means of a button on the left side ; directly over this is a 
window intended for the Bertrand lens to observe the axial figures of 
interference. 
The upper extremity of the tube carries the analyser with divided 
circle. The mounting of the Glan-Thompson prism is easily removed 
to introduce the oculars. These are provided with a reticule and a 
small screw which ensure the invariable position of the crossed threads 
in relation to the principal sections of the Nicols. The mounting of 
the upper analyser is also provided with an opening intended to receive 
the feather edges. 
M. Amann, who describes the Microscope, has been able to assure 
himself, by actual working, that the instrument possesses qualities of 
high precision, solidity, and simplicity, which render it an eminently 
practical instrument. 
Hew Attachable Mechanical Stage. — Fig. 44 is an illustration of 
Baker’s new mechanical stage, designed by Mr. Allen, and exhibited by 
Mr. C. L. Curties at the meeting of March 16th last. This apparatus 
can be easily attached to any Microscope that lias a rectangular stage, 
by means of two thumb-screws ; one of these, at the lower end, can be 
seen in the figure, but the one at the top is not shown, as it is below 
the stage. This attachment at both extremities gives the apparatus 
great rigidity. Both rectangular movements are performed by means 
of rack and pinion. The milled head seen at the right hand causes the 
transverse arm, upon which the slip rests, to move in a vertical direction 
over the stage ; but the principal novelty in this device is the method 
by which the transverse movement is controlled. Above the slip a 
transverse arm, fitted with a rank-and-pinion movement, is pivoted to 
the vertical dovetailed slide, and to its extremity is fixed a small piece 
of cork, which is kept in pressure against the top of slip by a spring 
placed on the right-hand side of the pivot. The friction of this cork 
pressing on the top of the slip is sufficient to impart transverse move- 
ment to the slip, when the milled head is turned. To diminish the 
friction between the bottom edge of the slip and the lower transverse 
arm, it is so arranged that the slip bears only on two points. Both 
