DU. A. E. H. TUTTUN ON TJIE ELA8MOMETEE, 
I GO 
The column has Iaco curved arms cast rvith it, on opposite sides of the axis; one 
l)8ars tlie microscope and the other a compact counterpoise for the latter. The 
microscope arm is a double one and the joarts are of T- shaped section to give rigidity; 
it terminates in a vertical hollow cylinder in which the microscope is capable of 
vertical motion by means of the usual rack and pinion. The magnification provided 
is about 15 diameters. The eye-piece is carried in an inner sliding tube which affords 
an adjustment for the focussing of the rectangular spider-lines ; the outer tube in 
which it slides screws into the carrier for the latter, which in turn slides tightly in 
the main oj^tical tube, and is provided with a flange by means of which the azimuth 
of the spider-lines may be adjusted. 
Tlie rectano’ulai' measuring movements liave been constructed in a manner which 
ensures both rigidity of their ])arts and complete absence of backlash in the working 
of the screws. The total thickness of the two sliders and their beds amounts to 
G centims. Provision for regulating the fit of the sliders about the beds is made in 
each case by making one of the guides of the slider take the form of a thick bevelled 
rod inserted between the correspondingly bevelled side of the bed and an outer flange 
of the slider provided to contain it, and slightly larger than the gajj between them, 
instead of cutting tlie bevel out of the slider itself on this side, as is done on the other 
side. The pressure of the bevelled rod against the bed is determined by two fixing 
screws and a regulating capstan screw between them. Tlie two capstan screws can 
also be used to fix the movements in any desired position. 
The measuring screws, which lie longitudinally along the centre of the beds and are 
mounted in closely fitting and very rigid liearings, have been cut with the utmost 
attainaljle accuracy. They have a pitch of I millim. A finer screw than this was 
undesirable, in view of the distances to be measured. The length of traverse provided 
in the case of each screw is about 33 millims., just enough to enable the two end¬ 
points of either knife-edge to be lirought into tlie centre of the field of view of the 
microsco])e. 
In order that there slioukl lie no backlash between them and the nuts attached to 
the sliders which they drive, a special device lias been adopted. It consists in 
making each of the nuts in two iiarts, which are forced continuously apart by a strong 
spring. The double nut is cylindrical in shape, with a broad stud projecting above 
by which it is fixed to the slider. The cylinder has an internal bore of about twice 
the diameter of the screw, extending from one end along three parts of its length ; 
the remaining quarter forms one part of the nut, being bored and tapped with a screw 
thread corresponding to that of the screw. The other part of the nut slides like a 
bored and tapped stopper into the open end of the cylinder, and, being of the same 
length as the jiart which is solid witli the cylinder, fills up the outer thu d of the large 
bore. The outer end of the stopper-nut has two steadying pins projecting radially at 
the opposite ends of the same diameter, which fit in notches cut to receive them in 
the cvlinder end, so that rotation of the stopper-nut about the cylinder and the nut 
