1894.] 



on the Newtonian Constant of Gravitation. 



365 



neously. S x is a micrometer screw-head, which is employed to push 

 the scale bodily to the right by measured amounts. The two micro- 

 scopes 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 focussing slide is then withdrawn, leaving them relatively unchanged, 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 frac- 

 tional 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 sus- 

 pended, 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 



Fig. 4. 



purpose. It is sure to be prismatic, and when inserted between the 

 microscope and an object it will certainly cause it to shift its ap- 

 parent 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 experimen- 

 talist. 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 experi- 

 mental 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 



