On Spectroscopic Measurements. 



281 



This curve, together with the mean of 

 estimates, is given in fio\ 2, Woodcut. 



a number of eye- 

 From these the 



following table of corrections may be obtained 



V.. 



•00 

 •05 

 •10 

 •15 

 •20 

 •25 

 •30 

 •35 

 •40 

 •45 

 •50 



Cor. 



•00 

 + •03 



+ •04 

 + •03 

 + •02 

 •00 

 -•03 

 -•05 

 -•07 

 -•08 

 -•10 



v e . 



Cor. 



•55 



-•12 



•60 



-•14 



•65 



-•15 



•70 



-•16 



•75 



-•16 



•80 



-•14 



•85 



-•13 



•90 



-•11 



•95 



-•08 



1-00 



•00 



The curves show a general tendency to estimate the visi- 

 bility too high when the interference-bands are clear, and too 

 low when they are indistinct. This tendency may be modi- 

 fied by a number of circumstances : thus, it increases with 

 the refrangibility of the light used ; it is greater when the 

 field contains a large number of bands than when there are 

 but few ; it is greater while the visibility-curve is falling 

 than when it is rising ; it does not seem to be greatly affected 

 by the intensity of the light ; finally, it varies on different 

 occasions and with different observers. Notwithstanding 

 these disturbing causes, the result, after applying the correc- 

 tion, will rarely be in error by more than one tenth of its 

 value, and ordinarily the approximation is much closer than 

 this*. 



* The formula for visibility deduced in the preceding paper is 

 ™ C 2 +S 2 



in which G = \<fi(v) cos kxdx> 



S = \(j)(z) sin hocdx > 

 F=fo(x)dx, 



D= Difference in path, 



and (f>(.v) represents the distribution of light in the source. 



In this expression no account was taken of the effect of extraneous 

 light, and it was assumed that the two interfering pencils were of equal 

 intensities. It can be shown that the error due to both these causes 



