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 FIG. 3. MIRROR dition, for I have found that the torsional 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 indiarubber 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, by means of which I could draw up a diaphragm with anything that might have fallen upon it. I have already stated that two measurements, viz. the horizontal distances between 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 attainable, for on these the result 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 arrangement which rests upon 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, and can be separated by a screw cone, forming a very delicate fine adjustment. This is operated by the screw-head S1; S3 is a focussing screw giving a fine adjustment to the focussing slide. S, S2 are two parallelising screws, the purpose of which is to bring the microscopically-divided glass scale into focus at each end simulta neously. S, is a micrometer screw-head, which is employed to push the scale bodily to the right by measured amounts. The two microscopes are focussed upon, say, the right sides of the wires, the focussing slide is then withdrawn, leaving them relatively unchanged, and the microscopic scale is then put in its place. The distance from wire to wire is thus transferred directly to the scale, and the fractional part of any one division of 1/100 inch is all that has to be referred to and measured by the screw. Every slide in this apparatus is geometrically arranged, so that the movements are all perfectly free, unconstrained and without shake. In measuring the distance between the fibres, which must be done while they are freely suspended, so that a force of a millionth of the weight of a grain will give them a considerable motion, means have to be provided to exclude draught, which yet must not interfere with the apparent distances of the fibres. No microscope cover-glass is any use for this purpose. It is sure to be prismatic, and when inserted between the microscope and an object it will certainly cause it to shift its apparent position. A piece of clear mica is perfect in this respect, no movement, even with a high power, being visible. I mention this as it well illustrates the sort of trap that is ever set for the experimentalist. If I had not been aware of this, and had used, as would be natural, a window of microscope cover-glass, then each fibre would have appeared as definitely in its place as before, but the place would have been wrong, perhaps by 1/1000 inch, and thus a consistent error affecting all the experiments alike would have been introduced, and no multiplication of observations or taking of means would have eliminated it. It is on this account that it is so important in experimental work to vary the conditions in every way, so as to discover unsuspected consistent errors. The microscope scale was made by Zeiss, and is a most perfect example of scale construction. In order to test the accuracy and find the errors of the scale, I took advantage of my visit to Cardiff for the FIG. 5. meeting of the British Association, to compare it with a series of Whitworth standard bars on Prof. Viriamu Jones's very perfect Whitworth measuring machine. For this class of work sunshine or dust give great trouble, but I was fortunate in having splendid weather for my purpose, as visitors will probably remember: it rained without ceasing during the two days I was making these measurements. Having now very imperfectly described the apparatus and the place in which I have carried out my experiments, I will next show a series of photographs, which I took by magnesium light, to give a better idea of the appearance of the apparatus and its surroundings. Fig. 5 is a view of the vault showing the clock, the eye end of the big telescope, and the little telescope. In the distant corner is seen the felt screen with a long slit, through which the scale and telescope can be seen from the mirror of the instrument. This, of course, is |