302 



H. NAGAOKA. 



Fin-. 1. 



Fig. 2. 



means of n telesoope placer! at T, and s(^ fc^ciissed that the ohserver 

 sees a, distant ])()int ]J (Fio-. 1) ; in other words, D is the so-caDed 

 diffraction ])oint. 



In order to find the a'eneral expression for tlie intensity of light 

 after it is diffracted hy an a])ertnre on a cnrved siirfjice, T shall assnme 

 tliat the diffracting apertm-e is very small compared with its distance 

 from the source of light, and from the ])oint at which the intensity of 

 diffracted light is ciHisidered. Conserjuently the amplitude of N'ihraiion 

 of the hght coming from different ])oints c^f the aperture will not ^ ary 

 at the ]ioint considered. 



I shall first discuss Fraunhofer's (telescopic) diffraction pheno- 

 mena. Referring to Fig. 1, let the vihration at L he represented hy 

 eus 2r ^r ; tlien, at any ])oint P on tlie diffracting a])ertiu'e, it will 

 he proportional to 



No\Y considering the ray in the directi(on DP, tlie vihration at anv \)ohi\ 

 C in the line FT due to tlic small element da at /' is propoi-tional t<» 



