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



which is the same as obtained with rectangular aperture of 

 height d. unaffected by absorption, save that the absolute 

 intensities at every point in the pattern are diminished in the 

 ratio 



L oo l o " /07\ 



T _ 1F (2 ° 



Since the intensity in the transmitted wave-front at the 

 thin edge of the prism is the same as in the incident beam 



(7=0). we have, by making ,i'=— - in (6), 



i Q 2 e m =l, or to 2 = e- Bb (28) 



Also if we choose a in (25) such that the greatest width of 

 the diaphragm is equal to d, we have 



Bb 



J =ae- . 



or a 2 =d 2 e- Bb (29) 



Hence we have from (27), {'28), and (29) 



loo 2 



I^ : 



— p-Bb 



(30) 



For the Bruce spectroscope before considered the value of 

 BJ> for wave-length 3900 is, as already computed, 



B6^6-59; 

 .-. ^ — -000 002. 



Although so much light is lost in diaphragming the aperture 

 as above described, the intensity in the resultant spectral 

 image will compare somewhat more favourably with that 

 secured under the actual conditions of absorption with full 

 aperture, because the " width " of the line is so much reduced 

 when the diaphragm is used. The intensity at the centre of 

 the diffraction-image with full aperture bd (see 10) is 



, r2 _.o^ 2 e™ + e-*°-2 . 



a lu -to Jya- (B[)) , .... {61) 



The ratio between this intensity and the intensity of 

 the image formed by the same instrument, unaffected by 

 absorption, will be 



T 2 ,,-lV/ 



I 2 ~(B&?* +e ~ s ' ' ' * ^ 3 ' 



