Absorption on the Resohina Power of Prism Trains. o69 



For regions of the spectrum lying below wave-length 3900, 

 materials having a smaller coefficient of absorption than light 

 flint or even crown-glass must be used. In the case of 

 glass, indeed, we soon reach a limit beyond which a further 

 reduction in density will be actually disadvantageous. As 

 will be seen from (2 L) and (23) we reach an absolute maximum 

 for E^ when the quantity 



In 



becomes an absolute minimum. For the various flint glasses 



already considered, the absorption-coefficient ft decreases 



dn 

 more rapidly than the dispersion-coefficient yr ; hence the 



quotient of these two coefficients decreases with diminishing 

 index of refraction. 



But as we pass from the flints to the crowns the reverse 

 takes place, and the value of (3 decreases less rapidly than 

 the dispersion. Hence we soon reach a point at which the 

 quotient 



n dX 



f3 



in 



is an absolute minimum for glass. Thus for the crown 0*203 



examin 



H line 



examined bv Yogel we have in the neighbourhood of th 



7i 3 9oo^r 1*532, ^Z:1650, and /3 ZT'054. 



Hence for crown 0*203 



&Q =0-000032, 

 an 



which is nearly 25 per cent, larger than the corresponding 

 quantity (0'000026)"£or the light flint 0*340. Hence, in spite 

 of its diminished absorption crown-glass would not be as good 

 a material to use for the prisms as light flint. 



We might perhaps find a flint for which the quotient of the 

 two coefficients is somewhat less than in the case of 0*340. 

 It is, however, safe to say th,at the value of this quotient 

 cannot be much reduced, and that the maximum practical 

 resolving-power which we can attain in spectroscopes with 

 glass prisms will not exceed 250,000 units in the photographic 

 region of the spectrum, nor 400,000 in the visual region. 



In order to attain still higher resolving-powers we must 

 use other materials than glass. Of those available for this 



