426 Prof. Carl Bams on ] nterferometry 



the last quantity is relatively small. The two terms o£ this 

 equation for e=l cm. show about the following variation : — 



Line... B. D. E. F. G. 



_ -oill - -0052 -0 + -0048 -0132 



- -000009 - -000004 -0 + -000005 -000013 



Hence for deviations even larger than a = 3°, the path 

 difference does not differ practically from the path difference 

 for the normal ray. 'Jhus it follows that the equation 



nX = — 2y + 2^/x/cos R 



is a sufficient approximation for such purposes as are here 

 in view. 



Finally, for e— '68 cm. the actual thickness of the plate 

 of the grating, y and N, the semi-path difference and the 

 displacement of the opaque mirror, will be : — 



D. 

 1-1660 



-•0018 



+ •0007 



- -0025 



where SN is the colour correction of A?/ , and AN = Ay — 8N 

 determines the corresponding displacements of the opaque 

 mirror ; or, more briefly, 



N =y — efi tan R sin R = 2e/ju cos R. 



The data actually found were (the B line being un- 

 certain) : — 



^0 = 



B. 

 11630 



^2/o = 



-•0033 



<5N = 



+ •0014 



^0 = 



- -0052 



T* 1 



•1668 



F. 

 1-1686 



G. 

 1-1713 



•o 



+ 0016 



+•0045 



•0 



- -0005 



- -0018 



•o 



+ •0021 



+ •0063 



Lines 



B. 



D. 



E. 



F. 



AN = 



- -0153 



- -OOi'8 



♦0 



+ •0057 



These results are again from 2 to 4 times larger than the 

 computed values. True the glass on which the grating was 

 cut is not identical with the light crown glass of the tables ; 

 but nevertheless a discrepancy so large and irregular is out 

 of the question. It is necessary to conclude, therefore, that 

 here, as in paragraph 8, the assumption of a total path 

 difference zero for the centre of ellipses is not true. In 

 other words, the equality of air-path difference and glass- 

 path does not correspond to the centres in question, it is 

 now in place to examine this result in detail. 



10. Divergence per Fringe. — The approximate sufficiency 

 q£ the equation 



n\=2eti/cosR,-2y (12) 



makes it easy to obtain certain important derivatives among 



