t)r. G. J. Stoney on Microscopic Vision. 423 



above 1*8 and negative below this frequency, at 45 volts it 

 was negative at all frequencies ; at 35 volts, however, it was 

 positive at frequencies above 1*8, and also at frequencies 

 below 1*8. That the resistance would be positive at all fre- 

 quencies seemed probable, since the curves connecting the 

 P.D. and current at constant arc-length for such carbons 

 show that with small arc-lengths an increase of P.D. is 

 accompanied by an increase of current. 



The experiments have been carried out in the Physical 

 Laboratory of the Central Technical College, and our best 

 thanks are due to Professor Ayrton and Mr. Mather for 

 much kind advice during the course of the work. 



Note. — Fluctuation of the Current given by a Continuous- 

 Current Dynamo. 



An attempt was made to replace the cells in Method I. by 

 a dynamo, but it was found that even with the alternator at 

 rest the electrostatic voltmeter showed a large deflexion. This 

 was evidently due to the oscillation of the current owing to 

 the commutator of the dynamo having a finite number of seg- 

 ments. The oscillation was found to increase as the brushes 

 were moved out of the sparkless position. The dynamo was 

 a 5 kilowatt 2-pole machine, and was giving 10 amperes at 

 70 volts. By adjusting the brushes to the sparkless position 

 this oscillatory current, measured in the same way as in the 

 case of the hissing arc above, could not be reduced below 

 2*5 per cent, of the continuous current, while by rocking the 

 brushes out of this position it could be increased to as much as 

 9 per cent, without the sparking at the brushes being excessive. 

 The commutator was in good condition. This oscillation may 

 introduce errors in the measurement by polarized instruments 

 of the currents given by dynamos owing to the differences 

 between the mean and the R.M.S. of such a current. 



XLII. Microscopic Vision. By G. Johnstone Stoney, 

 M.A., B.Sc, F.R.S. ^ 



[Continued from p. 349.] 



Addition to Part I. 



[The following section should come into Part I. after Theorem 1, be- 

 tween § § 7 and 8.] 



7 (a). Extension of Proposition 1. — In the proof of Pro- 

 position 1 it is assumed that the light emanates from each 

 individual point of the object iu an uninterrupted train of 



2H2 



