102 



S. R. Williams — Ah Achromatoscope. 



Fig. 1. 



=E=1 



EEE 



**k-^~-=\ 



=2 



w - 



Xs 



=1 



o==^ 



— O 



m J= 



— -::BB«| 



■ ^•~— — 





**== 



= ll» 





H 





Hi 





Hi 





r 





1, .. .:, 



a, 11. 





HP 



EH 



„,'^ 





1 



r ; cT 



the difference in path between the two 

 interfering rays is 4/2 times the wave 

 length past which the center of the 

 band is moving. Tims for successive 

 light bands, which move through the 

 spectrum, we need only to multiply a 

 half wave length by an even number to 

 get the difference in path for that par- 

 ticular wave length at which the center 

 of the band comes. For the light bands 

 in general, we have the relation holding: 



2pd = 2NV2, 



(2) 



where the various letters have the same 

 significance as in equation (1). 



When the surfaces are sufficiently 

 separated a continuous spectrum will be 

 lacking certain wave lengths for which 

 the difference in path of the two inter- 

 fering rays is an odd number of half 

 wave lengths. The resulting spectrum 

 is known as a channeled spectrum (see 

 figs. 1 and 2) and may be applied to 

 various optical measurements. 



To confine our attention more defi- 

 nitely to some particular point in the 

 spectrum, say X D , it will be noted that 

 by counting the number of light bands 

 which pass X D we have at once the par- 

 ticular order of interference for X D , 

 if we started to count with the first 

 light band which pushed in from the 

 violet end. If N D is this order of inter- 

 ference for X D and we could see all of 

 the bands lying between X D and X*, the 

 number would also be N n . This may 

 be expressed by the formula, 



N D -N. = N D 



(3 



which says that the total number of 

 bright bands passing X D is equal to the 

 number of bands in the spectrum be- 

 tween the wave lengths, X D and \». 

 Let the two optical surfaces be still 

 farther separated so that more bands 

 push along toward the red end, then N'd, the order of inter- 

 ference for the band now resting at X D , will be equal to the 



