118 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



S 



G, the lines of which are parallel to the sides of the slit. 

 Concentric with the middle line of the grating let there 

 be placed a circular zone or screen, Q', Q", Q'", Q"", of 

 white paper, through which there is an opening at A, to 

 admit the intromitted beam. 



A beam of parallel rays passing along S G will give a 

 bright image of the slit S when it impinges on the screen 

 at A'. This is the image by transmission. It would 

 also give another similar image at A, were it not for the 

 opening arranged there. This is the image by reflection. 

 Also from G, as from a central axis, there falls upon the 

 cylindrical paper zone, covering its surface all over, an 

 infinite number of radiations. 



These effects are seen with much more precision if 



