Energy Distribution of Diffraction Gratings. 893 



(1st class spectra) reflected by the oblique edges of the 

 grooves. If one edge of the groove makes an angle of 20° 

 with the original surface, and the light is incident normally, 

 the blaze of light, or the oblique image as we will term it 

 hereafter, is seen at an angle of 40°. In the case of the 

 spectrometer readings, it will be found 20° from the central 

 image, since, when the grating turns with the circle through 

 a given angle, the ray turns through the doable angle. The 

 angular position of the oblique image on our chart (fig. 3) is 



Fier. 3. 



3J* 30° IS' ao e IS 9 '0° 5° Cft^^te S' 



therefore given by the angle of the edge of the groove. It 

 has been indicated by brackets. In the case of very coarse 

 rulings it is merely an image of the source, but slightly 

 diffused by diffraction ; with the finer rulings we have the 

 broad maxima and minima of the first class (the positions of 

 the superposed patterns due to the individual reflecting strips 

 or the so-called oblique images, are indicated by brackets 

 in fig. 3). 



