;66 



NATURE 



[August 9, 1894 



B OS THE NEWTONIAN CONSTANT OF 

 GRAVITATION} 

 II. 

 HA\E already staled that two measurements, viz. the hori- 

 zontal distances between the axes of ihe wires which support 

 the lead la!!<, nni of the fibres which .-uppoit the gold balls, 



I 



Fio. 4. 



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 cooi- 

 pass, which is illustrated in Fig. 4. This is an arrangement 



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

 other of three grooves. The two traversing slides are drawi> 

 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 S4 ; S3 is a focussing screw giving a fine 

 adjustment to the focussing slide. s^ So are two p.iralleU 

 ising screws, the purpose of which is to bring the micro- 

 scopically-divided glass scale into focus at each end' 

 simultaneously. 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 side? of the wires, the focus'iing slide is then withdrawn,) 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 ihus- 

 iransferred directly to the scale, and the fractional part of any- 

 one division of i/ioo inch is all that has to be referred to anrt 

 me.isured by the screw. Every slide in this apparatus : 

 ijeometrically arranged, so that the movements are all perfecily 

 free, unconstrained, and without shake. In measuring the dis- 

 t.ince 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 dis- 

 tances of the fibres. No microscope cover-gl.iss is any 

 use for this purpose. It is sure to be prismatic, and whei» 

 inserted between the microscope and an object, it will certainly 

 cause it to shift its apparent position. A piece of clcir 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 wnuM he natural, a window 



Fig. $. 



which rests upon the lid of the Cfparatus on the circular V- of microscope cover-glass, then each filire w(juld have appeatf' 



grooYe seen in Fig. I, so that it can rotate without shake, as definitely in its place .as before, but ihc place would hav.' 



Upon Ihe lower framing rests the focussing slide, and on this a been wrong, perhaps by i/ioooinch, and thus a consistent erti't 



' ConiinueJ from p. J34. aflcctingall the experiments alike would have been introducec! 



NO. 1293, VOL. 50] 



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