A NEW INTERFERENTIAL FORINI OF ELASTICITY APPARATUS. 
1(51 
which is solid with it is prevented. Tlie middle widely bored part ot tlie cylinder 
accommodates the spiral spring. In fitting the arrangement, the loose or stopper-nnt 
is pressed into its place and compresses the spring; the screw is then inserted. This 
immediately secures the two sections together, and on removing the finger-pressure 
the spring exerts its force and pushes the loose nut away as far as any trace of 
backlash in the fitting of screw and nuts permits. In all positions of the compound 
nut along the screw, the latter is thus gripped between the two sections of the nut, 
and all possibility of backlash l^etween the screw and the slider carried above and 
fixed to the cylinder is eliminated. 
The guiding bed of the front-and-back movement carries on the left side a silver 
scale graduated in millimetres, and that of the right-and-left movement carries a 
similar one along its front edge. Each slider carries a little silver plate on which the 
indicating mark is engraved, and whicli is arranged to slide closely against and 
partially over the graduations on the scale. The microscope is made to read 
accurately to a thousandth of a millimetre in each of the two rectangular directions, 
hy a method "which entirelv avoids the use of an additional micrometer screw. The 
screw shaft in each case carries, between the bearing from which it emerges and the 
milled head by which it is rotated, a drum 8 centims. in diameter, fitted with an 
outer tyre of silver 2'8 centims. broad. This silver cylinder is engraved with eleven 
equidistant circles parallel to each other and to the cylinder ends, the two outside 
ones being only a millimetre distant from the latter. The nine inner circles thus 
divide the cylindrical surface up into ten circular strips 2'5 millims. broad. The two 
outside circles have each been divided into 100 equal parts 2'5 millims. long, and 
every alternate division of that edge-cii'cle neai'est the bearing has been engraved 
with its number. The division of these edge-circles has been done so that a line 
drawn parallel to the shaft axis frf)m any division on one of them would meet the 
other at a division. A hundred })arallel lines have then been drawn obliquely across 
the cylinder from each division of the numbered circle to the preceding division on 
the other edge-circle, thus covering the cylindrical surface with 1000 parallelograms. 
The space between any two divisions of the numbered circle is in this way sub-divided 
into ten parts by the intersections of the oblique line with the ten other circles, and 
an indicator in the form of a straight line parallel to the axis of the cylinder, 
suspended close above the lattei', enables the observer to read oft directly at which 
circle the intersection corresponding to the position of the microscope occurs. This 
indicating straight line is engraved on a glass plate and coloured red to distinguish 
it from the lines on the cylinder. The glass plate is fixed in a rectangular frame 
supported horizontally just over the surface of the cylinder, by a l)racket standing 
up from the end of the guiding bed. The red line has numbers from 1 to 10 also 
engraved against it and coloured red, exactly over the several circles in turn which 
follow the numbered circle next tlie bearing. It will be obvious that the position of 
the microscope at any time, along either of the two rectangular directions, is afforded 
VOL. CCIT.-A. 
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