158 
1)E. A. E. H. TUTTOX ON THE ELASMOMETEE, 
rectangular axes, passing through a brass sphere which each fork half encircles. The 
shaft is loosely sujiportecl near its front end in a stout ring carried at the head of a 
strong gun-metal tuliular column, fixed near the front edge of the basal slab. Tne 
rino- has a short stem rotatable in the bore of the column, hut made incapable of 
vertical motion by groove and pins. The front end of the shaft is notched to gear 
with the same milled head that fits the head of the endless screw itself The length of 
shaft is adequate to permit of the full extent of racking of the balance in or out of 
the recess between the blocks, so that whatever the position of the balance the control 
apparatus is equally available. 
During an actual determination, after the final adjustment of the pressure-point to 
the centre of the plate, experience shows that it is of some advantage to the attainment 
of absolute steadiness in the motion of the hands to substitute for the Hooke s jointed 
shaft a simple rigid shaft which is also provided ; it is fitted with a tightening collar 
at the end of tlie barred bore, where it grips the endless screw head, to prevent the 
slightest chance of slipping. Being twice as long as the jointed shaft, its front end 
is nearer to the observer, and is supported ni a sjDecial bearing fixed on the small 
mahogany base near the telescope pedestal. 
In order to render the control still more delicate, so that the bands shall march 
past the cross-wires with great deliberation, rendering their counting a very easy 
matter, the milled head can be replaced by a handle at the end of a long radial lever. 
To the end of a short thick tube, provided with inner cross-bar to fit the shaft notch 
and also a tightening collar to firmly grip the shaft, a strong plate is fixed, out of the 
front of which a sliding bed is cut lor the lever to slide in; by means of a longitudinal 
groove in the latter and a clamping screw, the lever can be fixed to the plate in any 
position, and thus the radius of the circle described by the ebonite handle can be 
modified as desired. When the lever is at its full length the raaius is 10 centims., 
and the circumference of the circle 63 centims. As one complete rotation corresponds 
to a rise or depression of the control screw, and therefore of the control jioint on the 
beam, of 0‘0035 millim., one-tenth of this, namely 0-00035 millim., corresponds to a 
movement of the handle of 6-3 centims. The pressure-point, however, moves through 
twice the distance that the control-point traverses, so that a movement ot 
0-00035 millim. on the part of the pressure-point corresponds to a movement of 
aliout 3 centims. on the part ot the handle manipulated by the observer. Hiis, 
however, is a very large amount ot movement, ot which a tenth part is still a very 
apprecialile quantity. 
It so happens that the half-wave-length of the red C-hydi-ogen light which the 
author usually employs is 0-00033 millim. As a change in the thickness ot the air- 
film equal to half a wave-length corresponds to the interval between the passage ot 
two consecutive interference hands past the reference spot, it will be clear that the 
handle has to he turned thi'ough a distance of 3 centims. for each transit of a whole 
hand, and therefore that the transit can be controlled with a high degree of delicacy. 
