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



purpose quartz, rock-salt, and fluorite are among the best. 

 Unfortunately I have not been able to find any reliable data 

 on the coefficients of.' absorption of these materials for different 

 wave-lengths. Although they are considerably less than 

 those for glass for both very short and very long wave-lengths, 

 they are sufficiently large to exercise an appreciable effect on 

 the resolving-power in these ultra-visible portions of the 

 spectrum. It is also necessary to remember that certain 

 kinds of glass, as well as other materials of which prisms may 

 be constructed, frequently show strong selective absorption ; 

 and certain regions even in the visible part of the spectrum 

 where this occurs may exhibit a lack of apparent definition 

 in consequence. In fact, in all discussions involving questions 

 of resolving-power, the possibility of the effects of large local 

 or general absorptions must be kept in mind. 



(Jj) The second effect of absorption which we have to con- 

 sider is that on the general form and apparent " width " of 

 the individual spectral lines. This effect is in many respects 

 more pronounced and striking than that on the resolving 

 power just considered ; for as will be seen by an inspection 

 of fig. 1 (PI. YIIL), the apparent width is increased in a 

 much greater ratio than the resolving-power is diminished by 

 increased absorption. 



As long as there is a well-marked minimum on each side of 

 the centre, we may consider the edges of the spectral image to 

 be defined by the position of these minima ; or, in other 

 words, consider that the apparent " width " of the line is 

 m m (fig. 1). For the value fi = this width for an infi- 

 nitely narrow slit and monochromatic radiation is of course 2x . 

 The effect of an increasing absorption is to increase all the 

 ordinates of the diffraction pattern relatively to the central 

 one (for a=0) ; but the ones in the neighbourhood of the first 

 minima are increased proportionally more than the others, so 

 that these points of minima are gradually obliterated, and 

 become indistinguishable for values of Bb greater than 1*5. 

 The edges of the line then become less and less sharply defined 

 and clearly marked. Their apparent position depends some- 

 what on the absolute as well as the relative intensity at 

 different parts in the diffraction pattern ; but in general they 

 may be taken to lie near the limits + m defined by the relations 



/(±»0=4«= 04I o; 



i. e. near the points at which the intensity of illumination in 

 the image first falls to less than 4 per cent- of the intensity at 

 the geometrical centre. 



Assuming this criterion, we have for the relation between 



