486 On the Visibility of Faint Interference-Bands. 



difficulty was obviated by the substitution for the selenite of 

 an accurately worked plate of quartz, cut parallel to the axis. 



The following were the readings by myself (R) and by my 

 assistant (G), when the bands were but just recognizable with 

 certainty. 





82 6 



72 13 



O i 



81 2 



71 59 





81 



71 40 



80 43 



72 16 





82 2 



72 40 



81 7 



72 9 





81 41 



72 









80 31 









Mean. 



..81 28 



72 8 



80 57 



72 8 



Hence 



(R) 2« = 9°20', (G) 2« = 8°49; 



so that, since sin^ 9° = '0245, the bands are visible when 1 — A 

 is less than half as great as before. The following were the 

 readings when the bands were considered to be still distinct : — 



83 15 



, 



70 33 



82 58 



70 52 



83 28 



69 53 



82 41 



71 2 



83 



70 30 



83 37 



71 20 



Mean... 83 14 



70 19 



1 



83 5 



71 5 



Hence 



(R) 2a = 12^55', (G) 2« = 12°0'. 



Here sin^ 12° 30'= •0372 ; so that a difference of 4 per cent. 

 between the darkest and brightest parts is sufficient to show^ 

 the bands with distinctness. 



It seems therefore that I was well within the mark in 

 assuming that bands involving 5 per cent, of ths brightness 

 mioht still be visible. 



