428 Michelson mid Morley — Wave-length of Sodium Light. 



Light from the source at s (fig. 1), a sodium flame, falls on the 

 plane parallel glass a, and is divided, part going to the plane 

 mirror c, and part to the plane mirror b. These two pencils 

 are returned along cae and bae, and the interference of the two 

 is observed in the telescope at e. If the distances ac and ab are 

 made equal, the plane c made parallel with that of the image 

 of b, and the compensating glass d interposed, the interference 

 is at once seen. If the adjustment be exact, the whole field 

 will be dark, since one pencil experiences external reflection, 

 and the other internal. 



If now b be moved parallel with itself a measured distance 

 by means of the micrometer screw, the number of alternations 

 of light and darkness is exactly twice the number of wave- 

 lengths in the measured distance; thus the determination con- 

 sists absolutely of a measurement of a length and the counting 

 of a number. 



The degree of accuracy depends on the number of wave- 

 lengths which it is possible to count. Fizeau was unable to 

 observe interference when the difference of path amounted to 

 50,000 wave-lengths. It seemed probable that with a smaller 

 density of sodium vapor this number might be increased, and 

 the experiment was tried with metallic sodium in an exhausted 

 tube provided with aluminum electrodes. It was found possi- 

 ble to increase this number to more than 200,000. Now it is 

 very easy to estimate tenths or even twentieths of a wave- 

 length, which implies that it is possible to find the number of 

 wave-lengths in a given fixed distance between two planes with 



