Diffraction by a Semi-infinite Screen. 



59 



was not found practicable to carry the measurements beyond 

 the third fringe. The photograph reproduced in Plate II. 

 fig. 4 (c) clearly shows that the contrasts between the 

 maxima and minima of illumination in this case are greater 

 than in the fringes of the Fresnel type. 



Two other cases besides that described above have been 

 investigated experimentally. In one of these shown dia- 

 grammatically in fig. 1 (ft), <p' was nearly equal to ir and <f> 

 was small and positive. The positions and magnitudes of 

 the maxima and minima of illumination were calculated from 

 SommerfekPs solution (the upper signs only being taken), 

 and the experimental data are shown in Table 111. for 

 comparison. The illumination curve for this case is shown 

 in fig. 2 (?>), and the photograph on which the measurements 

 were made is reproduced in Plate II. fig. 4 (6). In this 

 cas; j on the side of thn screen under observation, only the 

 reflected wave-front is limited by a boundary passing through 

 the edge of the screen, and suffers diffraction, and the 

 general agreement between experiment and theory is only 

 rendered possible by taking for the latter the rigorous 

 solution obtained by Sommerfeld, the boundary condition 

 5 = being assumed to be satisfied at the surface of the 

 screen. 



Table III. 



= 30-75 cm. X=4377 A.U. 0'=179°54''56. 



Calculated Intensity 



of the maxima 



and minima. 



Max. 383 

 Min. 018 



Max. 4*52 

 Min. 0-02 



Max. 338 



Min. 



Max. 

 Min. 



Max. 

 Min. 



Max. 

 Min. 



Max. 

 Min. 



Max. 



0-20 



3-20 

 0-21 



2-45 

 0-32 



1-17 

 0-60 



147 

 064 



1-39 



Calculated distance 



of the maxima and 



minima from the edge 



of the screen in cm. 



006 



•0140 



•020 

 •0278 

 •036 

 •0423 



•0490 

 •0558 



•060 

 •0(582 



•074 

 •079S 



•082 

 •0912 



•092 

 •1014 



Observeddistance of 

 the minima from the 

 edge of the screen. 



•0140 



•0421 

 ■0554 



•0681 

 •0796 

 •0907 



•1008 



