Measurement of the Obliquity Factor of Diffraction. 623 



are capable of experimental determination. The variation 

 of the obliquity-factor over the diffraction-pattern profoundly 

 modifies the distribution of illumination. In + he first place 

 the illuminations in corresponding bands on either side of the 

 central band, instead of being equal, become widely different. 

 The ratio of the illumination in two corresponding bands is 

 capable of photometrical determination and furnishes a 

 method of measuring the obliquity-factor. Then, again, the 

 curve of distribution of illumination in any particular band 

 of the pattern, i. e. from one minimum to the next, is 

 modified, and the position of the point of maximum 

 illumination is shifted. This shift furnishes a second method. 

 Both these lines of investigation have been worked on. 3n 

 the present paper only the first method will be dealt with, 

 the second being reserved for a separate communication. 



The ratio of the illuminations at corresponding points 

 (on opposite sides of the central band) is, as shown in the 

 paper quoted above (Phil. Mag. Jan. 1909, page 214), equal 

 to cos 2 1 1 cos 2 2 , where 6 X and 2 are the angles made by 

 the diffracted rays with the normal to the plane of the 

 reflecting surface or transmitting aperture. Putting 



*i='($-*i) and *,= (!-«,), 



the ratio of illumination may be put equal to &i 2 /c 2 2 } pro- 

 vided 6\ and 8 2 are fairly small. 



The ratio w r as determined experimentally by the method 

 of revolving sectors. Plate VII. fig. 2 shows the photo- 

 metric arrangement adopted. Two disks of blackened card- 

 board about 24 cms. and 21 cms. in diameter respectively, 

 are mounted concentrically, just touching each other on the 

 shaft of a small electric motor. The outermost annulus of 

 the first disk having a breadth of 6 cms. is divided into 12 

 equal sectors of 30° each and alternate sectors are removed. 

 The 2nd disk is similarly treated for a breadth of 3 cms. 

 from its circumference inward. It is arranged so that the 

 two disks can be clamped on the shaft in any desired position 

 relative to each other. On working the motor, the two 

 annuli (having a breadth of 3 cms. each) are seen to transmit 

 unequal intensities of light. The photometric disk lies and 

 rotates in a vertical plane. The diffraction-pattern is focusscd 

 on the photometric disk, so that the central bright band 

 (which, as also the other feebler ones on either side of it, is 

 vertical) lies on the edge between the two annuli at right 

 angles to the horizontal diameter of the disk. The broad 



