22 Messrs. Henderson and Henry on the Motion 



decreasing the amount of reflected light and increasing that 

 of transmitted light. By this means we obtained in the field 

 of the telescope perfectly defined interference -bands, the 

 slightest motion of which could be at once detected. 



If now the asther can be made to move in the direction 

 of the transmitted beam, the increase thus caused in the 

 velocity of light in that direction and the corresponding 

 diminution of the velocity of light in the contrary direction 

 should alter the difference of ph.'ise between the beams on 

 arrival at the mirror a, and so produce a displacement of the 

 bands. 



To endeavour to produce this motion of the asther our 

 apparatus was arranged as follows : — Two condensers, con- 

 sisting each of two plates of sheet tin (56 centim. x 39 

 centim.) separated by a glass plate (3 millim. in thickness), were 

 laid horizontally side by side on paraffin blocks on a table. 

 The bottom plates were connected to the terminals of the 

 secondaiy of a Ruhmkorff coil. The upper plates were 

 joined through a circuit containing two spark-gaps — one 

 placed at the slit of the collimator so as to be the source of 

 light, while the other was situated at a distance of 4 centim. 

 from the edge of the mirror c and in the plane of the mirror. 



The primary plates of the condenser were also connected 

 through a spark-gap. When these plates are raised to a high 

 potential by the coil they suddenly discharge themselves 

 through the spark-gap, thus causing the secondary plates to 

 discharge themselves through their connecting circuit with 

 its spark-gaps. 



The equation for the discharge of a condenser of capacity 

 C through a circuit of self-induction L and resistance li is 



cPQ dQ Q 



whence / R Vw-JK 



where Q is charge in condenser at time t after beginning of 



discharge. 



, 4L 

 If Rr < -p-, the discharge is oscillatory, and 



The amplitude of the oscillation is 



a 5. 

 Ae ~ 2L • t 



