80 Mr. A. Whitwell on the Lengths of the 



the images be formed at the lines B or H it will be seen that 

 the edges of the image bands due to the object bands 1, 3 

 and 2, 4 and 3, 5 coincide, and we shall get the central part 



of the screen almost uniformly illuminated from two slits 

 whilst the top and bottom will be illuminated from one slit 

 only. Again, if the images are formed at C or G the edges 

 of the image bands due to slits 1, 4 and 2, 5 coincide and 

 we get the centre of the screen illuminated by three slits, the 

 outer parts being illuminated from two slits and the edges 

 from one slit only. Similarly, if the images are formed at D 

 or F the edges of the image bands due to slits 1 and 5 

 coincide and the centre of the screen is illuminated from 

 four slits, the light fading off towards the top and bottom 

 edges as the screen is illuminated from three, two, and one 

 slit only. When the object is at a great distance all the 

 images coincide at F the principal axial focus. 



