BY A. h. McAlLAY. B.Sc. H.A.. ThD. 95 



given by the ordinate of the visibility curve, -and these two 

 beams will combine to produce illumination on the field. 



X> 



Tt<}ii.Ye yl 



If the distance between corresponding points of the two 

 apertures is I, the phase difference between the two beams 



is evidently - '^ Uin 6 (see Figure VI.), Thus the two 

 beams will completely interfere when rJT i ^ilI t) = tt '2 m-l) 

 where m is an integer, i.e., where -in - ' '" ~ • The 

 centres of the bright bands or spectra evidently come where 

 2 tt/X 1 ,siii<? -2 111 TT, i.e., where .-in 6> -l^-Jm — . The 



1:: I i 



spectra formed for m = l, 2, 3, etc., are cailed first, second, 

 third, etc., order spectra. If the value of sin = '-!!— is 



such that it coincides with the value for a minimum of the 

 visibility curve, the spectrum will have zero intensity. Ob- 

 viously in general, the intensity of the spectra will depend 

 on the position they happen to occupy in the visibility curve, 

 and this depends on the ratio of the pitch of the grating, 1, to 

 the width of the aperture, e. 



The abcve discussion has been concerned with aomo- 

 geneous light. In the case of white light, each colour forms 

 its spectra in a different place, determined by the value of 

 '\ ; hence the colour effects at the edges of the spectra. 



SECTION 7. GRATING WITH MORE THAN TWO 



LINES. SECONDARY MAXIMA, ETC. 



Widen the slit C till three apertures of D are opened and 

 observe the appearance of the pattern in the field of F. Next 

 uncover four apertures, then five and six. Finally, remove 

 the slit C so that the whole of D is effective. D should con- 



