92 iMioi'KKTiHs or orxn'AL chatinws 



V • 3 • 



2 \ '^n^ 



i 



/ 



/ 



A 



lw^»^ 



.<-. \(Si 



' — 5 



if \ 



Ftqure JST 



if there are n elements, the angle between the end v.'ctors 



will be '"'^"ai.' -ii.ia = a and as n ab = e— the width of 



,\ f 



the slit a= -"'' ''• Figure IV., 1, 2, 3, 4, and 5, show the 



.\ f 

 chain of vectors for a = o, n, 'iw. ^ir, itr. Remembering that 

 the resultant is the line joining the ends of the chain, it is 

 easy to see that the visibility curve must have the general 

 form found for it in section 3, Figure II. It must not be 

 forgotten that the intensity of the illumination is proportional 

 to the square of the amplitude of the wave, while the vector 

 represents the amplitude. It is evident from a consideration 

 of Figure IV. that the first minimum of the curve will occur 



when a -In which gives as found experimentally d = or 



e 



•! fin ^ X. A more complete theory (see Schu.ster, Theory 



of Optics, p. 99 et seq.) shows that the expression giving the 



form of the visibility curve is I -:: '.. f*""' '^'f 1 where I 

 is the intensity, I . a constant and a "" '• Hin tK 



