360 Prof. F. L. 0. Wadsworth on the Effect of 



is so chosen that the intensity 1| at the centre of the diffrac- 

 tion image, f=0, is the same (unity) in each case. The 



abscissae are expressed in terms of the angular coordinates — 



which are determined by the relations 



S X 



ex. 



and therefore 



The comparative values of 1^ for values of a. ranging from 

 to 6a (> and for the values of Bb given above, are tabulated 

 in columns 3, 4, 5, 6, 7, and 8 of Table I. (p. 359) and plotted 

 as full-line curves in fig. 1. (PL VIII.). For comparison the 

 value of I 2 for B = is also tabulated in column 2 of this table, 

 and plotted as a dotted curve in fig. 1. 



An inspection of this table, or the corresponding curves of 

 fig. 1, shows that the effect of increasing absorption is to 

 change the form of the curve I 2 =f(a), 1st, by gradually 

 obliterating the points of maxima and minima on each side of 

 the centre, and, 2nd, by gradually increasing its apparent 

 width mm. Vfe have to consider this change in form (a) on 

 the resolving-power of the instrument, (b) on the visual or 

 photographic appearance of the spectral lines. 



[a) The visual criterion for resolving-power is that two 

 lines of equal intensity may be considered as definitely 

 "resolved'"' when the illumination at the centre of their 

 superimposed diffraction images is not more than 0*81 of the 

 illumination at the centres of the images themselves. In 

 the case of an instrument unaffected by absorption (equation 

 11). this corresponds to a separation of the two lines by an 

 amount 



\ 

 o"o=«o= J 



as defined by (14). 



In the case of two equal lines separated by an angular 

 interval a the relative intensities in the physical imaoe of 

 the double source at the points corresponding to the centres 

 of the lines themselves and a point midway between them 

 will be 



I 2 9T 2 



.2 — p , T-> ..... (ID) 



