364 



Mr. C. Vernon Boiji 



[June 8, 



that the quartz fibre hanging from the cross-arm above may rest 

 definitely in them. The central hook is for the purpose of hanging 

 the " counterweight," i.e. a slender silver cylinder of exactly the 

 same weight as the gold balls with their fibres and hooks. By this 

 means the unknown moment of inertia of the mirror may be eliminated 

 with the fibre equally stretched in both cases, a most necessary con- 

 dition, for I have found that the torsional 

 Fig. 3. rigidity is seriously affected by variation 



in stretching. 



Means are provided by which I can 

 effect the transfer of the gold balls from 

 the beam to the side hooks or the reverse, 

 or change their places without opening the 

 window ; but these and numerous other 

 important details I must pass over. 



Unfortunately accidents are liable to 

 happen, and, as I know by dearly-bought 

 experience, the gold balls may sometimes 

 be precipitated down the central tube. I 

 have recovered them sometimes by an india- 

 rubber tube, let down through the window 

 aperture, sucking at the other end until 

 they closed the open end, when they could 

 be drawn up. Latterly I have made use 

 of a magnetised tuning-fork to pick up a 

 very small fragment of iron tied to a 

 silk line, b} 7 means of which I could draw 

 up a diaphragm with anything that might 

 have fallen upon it. 



I have already stated that two measure- 

 ments, viz. the horizontal distances be- 

 tween the axes of the wires which support 

 the lead balls, and of the fibres which 

 support the gold balls, must be made 

 with the highest degree of accuracy at- 

 tainable, for on these the resnlt directly 

 depends. In order to accomplish this I 

 had to design a special instrument, an 

 optical compass, which is illustrated in Fig. 4. This is an ar- 

 rangement w r hich rests ujjon the lid of the apparatus on the circular 

 V-groove seen in Fig. 1, so that it can rotate without shake. 

 Upon the lower framing rests the focussing slide, and on this 

 a pair of traversing slides, each carrying a microscope in one or 

 other of three .grooves. The two traversing slides are drawn together 

 by a spring, aud can be separated by a screw cone, forming a very 

 delicate fine adjustment. This is operated by the screw-head S 4 ; S 3 

 is a focussing screw giving a fine adjustment to the focussing slide. 

 S 2 S 2 are two parallelising screws, the purpose of which is to bring 

 the microscopically-divided glass scale into focus at each end simulta- 



