S. R. Williams— An Achromatoscope. 101 



Art. II. — An Achromatoscope* ; by S. R. Williams. 



The dark center of Newton's rings is explained as being 

 due to the interference of two rays of light reflected from two 

 optical surfaces so close together that only a half wave length 

 exists in the difference of path between the two rays and this 

 difference is due to change in phase at one of the surfaces of 

 reflection. In other words, the two surfaces are in contact 

 and the change in phase introduced at one surface is half an 

 undulation for all wave lengths as Young showed by means of 

 an oil of sassafras film between crown and flint glass surfaces. 

 For an air film then, the central spot of Newton's rings is 

 black for all wave lengths because we have destructive inter- 

 ference for a difference in path such that, 



2,*tf=(2N + l)^ (1) 



i. e., for an odd number of half wave lengths, where N" is the 

 order of interference, d the thickness of the film, /* the index 

 of refraction and A. the wave length. When white light, 

 reflected from this portion of Newton's rings, is examined 

 spectroscopically only a faint spectrum may be seen as a little 

 more light is reflected from the front surface than from the 

 rear of the film. 



(a) Effect of Separating the Two Optical Surfaces. 



If now the two optical surfaces are slowly separated an 

 interesting phenomenon occurs in the field of view of the 

 spectroscope ; light begins to push in from the violet end of the 

 spectrum in the form of a broad bright band which moves 

 toward the red end of the spectrum. For any particular wave 

 length in the spectrum past which the center of this broad 

 band of light passes, it means that the difference in path has 

 been increased by a half wave length or that the total differ- 

 ence in path is now a whole wave length for that particular 

 point in the spectrum, where the center of the band lies. This 

 also means that the two surfaces have been separated by a dis- 

 tance equal to a quarter of the wave length just mentioned. 

 As the surfaces are still farther separated the broad band keeps 

 pushing out toward the end of the spectrum where the wave 

 length is infinite, \ m . Other light bands but somewhat nar- 

 rower are following this first one. For the second light band 



*Read by title at the New York meeting of Amer. Pays. Soc. , Oct. 31, 

 1914. Abstract, Phys. Rev., vol. iv, p. 550, 1914. 



