58 Mr. S. K. Mitra on the Problem of 



The fourth column in the Table gives the positions of the 

 maxima and minima of illumination measured on a photo- 

 graph taken of the fringes due to a screen grazed by the 

 incident rays." They agree closely with the theoretical values 

 given in the third column. 



The ratios of the intensity of illumination at the maxima 

 and minima have been determined photometrically for com- 

 parison with the theoretical values shown in the second column 

 of Table I. As mentioned above, the intensities were found 

 to be independent of the plane of polarization of the incident 

 light. The method adopted for the photometric work was as 

 follows. The incident light was plane polarized by passage 

 through a nicol. Two narrow slits were mounted one above 

 the other in the plane of the diffraction fringes, and a thin 

 plate of mica of proper thickness ('032 mm.) was fixed up on 

 the upper one, so oriented that it circularly polarized the 

 light tailing on it. The field was observed through an eye- 

 piece and an analysing nicol mounted in a graduated circle. 

 The lower and the upper slit were then respectively set on 

 the first maximum and the first minimum, or the second 

 maximum and the second minimum, and so on. The illumi- 

 nation of the upper and the lower slit was equalized by 

 rotating the analysing nicol, and the position of the analyser 

 at which the light from the lower slit was extinguished was 

 also noted. The angular difference of the two positions 

 suffices to give the ratio of the illumination. Thus if 6 X and 

 2 be the two positions and I x and I 2 the intensities of illumi- 

 nation of the upper and lower slit respectively, 



Ij/I^sin 2 ^-^). 



A correction was made for the loss of light in transmission 

 through the mica sheet covering the upper slit. Table TI. 



Table II. 



-n , . „ . , ., r ~, -, Value calculated Value calculated 



Katio of intensity of Observed frQm Sommerfeld > s from Fresnel's 

 Minima and Maxima. value. expression. integrals. 



1st min. and 1st. max. 



■36 



•30 



•58 



2nd min. and 2nd max. 



•53 



•50 



•67 



3rd min. and 3rd max. 



•60 



•57 



•79 



shows the results of the photometric work which was carried 

 out on the fringes obtained with monochromatic light. It 



