456 Messrs. Michelson and Morley on the Relative Motion 



in front of the condensing lens, were made to coincide, the 

 telescope was now adjusted for distinct vision of the expected 

 interference-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 t ; then white light was restored, the screw altering the 

 length of path was very slowly moved (one turn of a screw 



Kff. 5. 



a 



b 



6 





b 



r 9 d 



9 c\ 



1 T a > Q 



i 





f 



I 







1 



i 



1 





i 



i 



i 





1 



i 



i 







of one hundred threads to the inch altering the path nearly 

 1000 wave-lengths) till the coloured interference- fringes 

 reappeared in white light. These were now given a con- 

 venient width and position, and the apparatus was ready for 

 observation. 



The observations were conducted as follows : — Around the 

 cast-iron trough were sixteen equidistant marks. The appa- 

 ratus 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 com- 

 pleted six revolutions. It was found that by keeping the 

 apparatus in slow uniform motion, the results were much 

 more uniform and consistent than when the stone was brought 

 to rest for every observation ; 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 



