Absorption on the Resolving Power of Prism Trains. 361 



and the lines will be resolved or not according as l m is less 

 or greater than 0*81 I c . 



Applying this criterion to the case of the images I 2 , shown 

 in fig. 1, we find for a constant separation o~ = ot the follow- 



I 2 



mo- values of -'" : — . 



I 



For B,= .5754:| =1 ^ = ^ + 



B/> = 1-38(52: 

 B6 = 2-1972: 

 B6 = 2'7726: 





•9054 

 1-0464 ~ 



•86 + 





1-0264 



1-1089 ~ 



•92 + 



if*"" 



1-1256 



1^1621) 7 



•97- 



P 1 o±94 



B.= 7-«25: ^ = g| =1-07 + 



which shows that the effect of absorption is to decrease the 

 resolving-power of the instrument by an amount depending 

 both on the value of B and of b. 



To find the separation necessary for resolution we must 

 select a value of a such that 



JV =0 :81. ..... (16) 



If we put 



and 



gB64.g-B6_2-.4D, 



(B'6) 2 = 4F 2 , 



7T O" 



a 2 



we obtain from (9) and (15) 



l7X rD D + sin 2 2x1 , E ™ . , N 



(F 2 + ^ 2 )[ F + F2+(2 ^ )2 j-^5(F 2 fsm^.f) ? • (10 



