Telescopes and Spectroscopes for Lines of Finite Width, 325 



that the intensity at the centre of the diffraction-pattern of 

 the double source (shown in full lines in fig. 2) should be 



Fig. 2. 



son 



about 0*8, the intensity at the maxima corresponding to the 

 centres of the two geometrical images. In order that this 

 may be the case the distance between these centres in the 

 three cases a=a, cr=2«, and cr = 3« must be for 



a = a, angular distance between centres = l*27a = cr -|- 0'27a, 

 o-=2*, „ ii ii ,, =2'2Lz= o - + 0-2L*, 



<r = 3«, „ „ „ „ =3-20«=cr + 0-20«, 



or in general „ „ „ = 2 = cr + S. 



From these and intermediate values the curve in fig. 3 

 (p. 326), which represents the relation between the angular 

 width of the lines and the angular distance 8 between the con- 

 tiguous edges necessary for distinct resolution, was plotted. 



In order to test these results experimentally a fine black 

 wire was stretched across the centre of an ordinary double 

 motion slit, thus forming two parallel slits whose widths 

 could be simultaneously varied (by opening the slit), while 

 the distance between the contiguous edges (which was equal 

 to the diameter of the wire) remained constant. The two 

 slits were uniformly illuminated by the light of the sun or an 

 electric arc passing through a screen of white paper, and were 

 viewed by a telescope over whose objective was placed a 

 rectangular opening of width b. 



The slit was set at various measured widths, and the dis- 

 tance of the telescope from it varied until the two halves of 

 the slit were just resolved. If D represents the distance of 



