382 A. A. MicheUon and E. W. Morley — Influence of 



branch again into two ; the two pairs being joined each to one 

 of the tubes. The branches were provided with large valves, 

 by turning which the current was made to flow in either direc- 

 tion through the tubes and into a large tank, from which it 

 was afterward pumped up to the upper tank again. The flow 

 lasted about three minutes, which gave time for a number of 

 observations, with the flow in alternating directions. 



Method of observation. — In the first series of observations a 

 single wire micrometer was used in the eyepiece of the observ- 

 ing telescope, but afterward a double wire micrometer was 

 employed. The tubes being filled with distilled water, the 

 light from an electric lamp was directed toward the central 

 glass of the refracto meter and the latter adjusted by screws till 

 the light passed centrally down both tubes, and then the right 

 angled prism at the further end adjusted till the light returned 

 and was reflected into the telescope, where generally two im- 

 ages were observed. These were made to coincide, and the 

 fringes at once appeared. They could then be altered in width 

 or direction by the screws, till the best result was obtained. 

 A slight motion of one of the mirrors produced an inclination 

 of the fringes, and the horizontal wire of the micrometer was 

 placed at the 'portion of the fringes which remained fixed, notwith- 

 standing the movement of the mirror. This adjustment was 

 frequently verified, and as long as it was true, no motion of 

 the tubes or distortion of the glasses could have any effect on 

 the measurements. During this adjustment it was found con- 

 venient to have a slow current of water, to avoid distortions on 

 account of unequal density. 



The signal being given the current was turned on, and the 

 micrometer lines set, one on each of the two dark bands on 

 either side of the central bright fringe, and the readings noted. 

 The difference between them gave the width of the fringe, and 

 their mean, the position of the center of the central white fringe. 

 This being verified the signal was given to reverse the current • 

 when the fringes were displaced, and the same measurements 

 taken ; and this was continued till the water was all out of the 

 upper tank. Following is a specimen of one such set of ob- 

 servations. 



No. 63. 



Direction of current, + 



Micrometer wire, 



I, 



r. 



11 



34 



13 



35 



10 



40 



13 



38 



14 



40 



10 



35 



I. 



r. 



80 



93 



71 



88 



73 



90 



67 



92 



65 



89 



61 



94 



Means 11-8 37*0 69*5 91-0 



