260 



Prof. C. V. Boys. 



[June 20, 



These figures are represented by the curve in fig. 3, which shows 

 the best position for an attracting mass at any distance from the 



Fig. 3. 



axis 0. The inclination of this with the line ON at the point N is 

 35° 16', or an angle of which the tangent is equal to l/\/2. This 

 curve also shows the best position from which a source of light at 

 any distance from would most brightly illuminate a small surface 

 at K lying along ON". 



If now an attracting ball is placed in a position of maximum effect 

 with its centre on this line it will act on the further suspended ball, 

 tending to deflect the beam in the opposite direction, and this will 

 become more marked as the distance between the centres of the 

 attracting balls increases, and so the increased effect which would 

 be due to a greater attracting ball may be largely compensated by 

 the increased action on the remote end of the beam. The azimuth 

 at which the maximum effect is produced is also changed. 



I have practically overcome this difficulty by arranging the two 

 sides of the apparatus at different levels. Each large ball is at or 

 near the same level as the neighbouring small ball, but one pair is 

 removed from the level of the other by about the diameter of the 

 large balls which in the apparatus which I have now the honour to 



