Earth and the Luminiferous Ether. 



339 



but finer; it also had an adjustment in the direction of the 

 incident ray, sliding forward or backward, but keeping very 

 accurately parallel to its former plane. The three adjustments 

 of this mirror could be made with the wooden cover in position. 

 The paths being now approximately equal, the two images 

 of the source of light or of some well-defined object placed in 

 front of the condensing lens, were made to coincide, the teles- 

 cope was now adjusted for distinct vision of the expected inter- 

 ference bands, and sodium light was substituted for white light, 

 when the interference bands appeared. These were now made 

 as clear as possible by adjusting the mirror e,\ then white light 

 was restored, the screw altering the length of path was very 

 slowly moved (one turn of a screw of one hundred threads to the 

 5 inch altering the path 



nearly 1000 wave-lengths) 

 till the colored interfer- 

 ence fringes reappeared 

 in white light. These 

 were now given a con- 

 venient width and posi- 

 tion, and the apparatus 

 was ready for observa- 

 tion. 



The observations were 

 conducted as follows : 

 Around the cast-iron 

 trough were sixteen equidistant marks. The apparatus was 

 revolved very slowly (one turn in six minutes) and after a 

 few minutes the cross wire of the micrometer was set on the 

 clearest of the interference fringes at the instant of passing 

 one of the marks. The motion was so slow that this could be 

 done readily and accurately. The reading of the screw-head 

 on the micrometer was noted, and a very slight and gradual 

 impulse was given to keep up the motion of the stone; on 

 passing the second mark, the same process was repeated, and 

 this was continued till the apparatus had completed six revolu- 

 tions. It was found that by keeping the apparatus in slow 

 uniform motion, the results were much more uniform and con- 

 sistent than when the stone was brought to rest for every ob- 

 servation ; for the effects of strains could be noted for at least 

 half a minute after the stone came to rest, and during this time 

 effects of change of temperature came into action. 



The following tables give the means of the six readings ; the 

 first, for observations made near noon, the second, those near 

 six o'clock in the evening. The readings are divisions of the 

 screw-heads. The width of the fringes varied from 40 to 60 

 divisions, the mean value being near 50, so that one division 



