449 



pursued identical, not only equivalent, paths, at least this is the 

 case for that part of the system of interference fringes wiiich in 

 white light forms the centre of the central hand. 



Fig. 1. 



Tn order to verify the formula (3) it is necessary that the ligiit 

 be monochromatic. Further it seems of immense advantage to have 

 a water current which remains constant during a considerable time. 



For observations with violet light this even becomes strictly neces- 

 sary, because visual observations are impossible with the violet 

 mercury line (4358) nsed. Michei.son obtained a flow of water by 

 filling a tank, connected with the apparatus ; by means of large 

 valves the current was made to flow in either direction through the 

 tubes. ''The flow lasted about three minutes, wliich gave time for 

 a number of observations with tiie flow in alternating directions". 

 In view of ray experiments the municipal authorities of Amsterdam 

 permitted the connection of a pipe of 7.5 cm. internal diameter to 

 the main water conduit. There was no difficulty now photographing 

 the violet system of interference fringes, though the time of expo- 

 sition with one direction of flow was between 5 and 7 minutes. 

 The pressure of the water proved to be very constant during a 

 series of observations; the maximum velocity in the axis of the 

 tubes, of 40 m.m. internal diameter and of a total length of 6 

 metres, w^as al)Out 5,5 metres. 



Before recording some details of my experiments, I may be per- 



