ON A QUARTZ THREAD GRAVITY IIAI.AM I |*J 



it with nickel, which for ordinary purposes may be regarded as incapable of amalga- 

 mation. The tit of the shafts both of the vernier arm and of the arrester is made 

 as good as possible without being at all stiff. The joints between the shafts and the 

 cylindrical holes in which they work are further filled in with tallow. Outside a 

 cup-shaped depression is arranged, and this is filled with mercury. As there is a 

 partial vacuum inside the apparatus, the tendency is for the mercury to drive the 

 tallow through the joint and then to follow it. The joint is very fine, however, and 

 the surface tension of the mercury tends to oppose this motion. The practical result 

 is that we obtain a smoothly working motion and a sufficient air tightness so far as 

 inwardly directed air pressures are concerned. 



Microscope. 



The microscope is supported on a flange soldered to the thick copper tube. There 

 is also a flange on the tube within which the microscope is fixed, and the adjustments 

 of the microscope are partly made by sliding one flange over the other, the clamping 

 screws which press the opposing surfaces together being passed purposely through 

 holes much larger than the screw diameters. The flanges are ground to fit, and are 

 put together with a little tallow for the sake of securing air tightness. 



The microscope itself is an ordinary Zeiss microscope tube furnished with the " A " 

 objective, giving a magnification of about 100 diameters. The adjustments are made 

 as follows. It is required to adjust the microscope so that the image of the end of 

 the lever will be sharply in focus, and bisect the cross wire when it is a small definite 

 amount below its position of instability. For the purpose of arriving at this adjust- 

 ment the object-glass is first soldered by its screw mounting to a length of brass 

 tulje which fits inside the stronger tube very well. The outer tube is attached 

 by soldering and screwing to the flange, and carries the eye-piece itself mounted 

 in a short length of tube in which the cross wires are fixed. The eye-piece 

 is focussed on the cross wires as usual, the object-glass is placed in position, 

 and the outer tube is levelled by means of the riding level. The focussing is 

 then accomplished by sliding the object-glass tube in or out, and when the 

 desired adjustment has been obtained, the flange is undamped and the object- 

 glass tube sweated in position with tinman's solder, by which means it becomes 

 rigidly attached to the outer tube. The outer lenses of the object glass are also fixed 

 in their mountings by means of wax, but the workmanship of the cell is so good that 

 the wax acts merely as a means of preventing the leakage of air. No doubt some 

 change in the position of the axis of collimation occurs as the temperature rises. This 

 will do no harm if the position of the axis of collimation can be regarded as a function 

 of the temperature, but the chance of irregularities must be put up with so long as 

 brass mounts are used : with platinum a greater degree of certainty is to be expected. 

 It is fair to say, however, that we have no reason to suspect the axis of collimation of 

 movement rather than any other part of the apparatus. When the preliminary 



