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Prof. R. W. Wood on Diffraction 



The spectrum seen in the direction A exhibited a dark 

 band in the red and a very faint narrow bright band in the 

 orange. Previous experiments with another grating having 



Fisr. 1. 



a. 



6. 



shown that lightly wiping the surface modified the appear- 

 ance of the bands, an attempt was made to better them in 

 the case of this grating. A very light rubbing with a 

 powder-puff made of swan's down increased the blackness 

 of the dark band and increased the narrowness and intensity 

 of the bright band, which presently with a little further 

 rubbing became a line. The lower corner of the grating- 

 was now rubbed lightly with chamois skin. This operation 

 obliterated all trace of the bright line, and caused the dark 

 band to shift into the yellow-green region. If the spectrum 

 was made to cover the entire surface of the grating by 

 bringing the source of light nearer, the dark baud in the 

 red was seen to pass gradually into the green region, the 

 band appearing curved, as shown in fig. 1 b. In this figure 

 the lower right-hand corner of the ruled surface has been 

 rubbed with chamois skin. 



The relations between the positions of the bright line and 

 dark band and the angles of incidence were now determined. 

 The circle was turned so as to rotate the grating in a clock- 

 wise direction (fig. la) through an angle of 11°. The dark 

 band moved from the red down into the green, preceded by 

 the bright line which remained distinct up to a rotation of 



