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THE ROYAL SOCIETY OF CANADA 



Theory and Results 

 3. Consider first the set of normal rays with the corresponding 

 central element of the spectrum. By drawing up M the distance 

 DJ2 the number of wave-lengths in the adjustable path is decreased 

 by D/\ for light of wave-length X. If ii is the corresponding index 

 of refraction of the glass plate its insertion introduces {{x—1) T/\ 

 wave-lengths. Hence the number of wave-lengths in the difference 

 of path is, 



n={D-{f.-l)T}/\ (1) 



For the constituents of the spectrum for which n is integral 

 there is reinforcement, and in passing from^ one integer to the next 

 the light intensity goes through a complete cycle of values. The 

 spectrum is, therefore, crossed by a series of fringes for which, in 

 general, the numbers of waves in the difference of the interferometer 

 paths differ by unity for each pair of adjacent fringes. Equation (1) 

 may, therefore, be taken to represent the distribution of these fringes 

 throughout the spectrum^, expressing n as a function of X, remembering 

 that jj, is also a function of X. 



Assuming the value of /j. for the Di line the number of Di wave- 

 lengths for the value of D corresponding to Plate 1 is 1131.8 and for 

 the other plates the values of n^,^ are shown in Table 1. With these 

 values as starting points one may pass up and down the spectrum and 

 assign the value of n to each fringe. The values of n increase from 

 both directions toward a centre, the values being equal for equal 

 counts on either side of that centre. Each negative shows several 

 hundred fringes, and Table 2 contains an illustrative selection from 



Table 2 



