530 Mr. A. E. Oxley on an Apparatus for the 



Table IT. 



Line. 



\ (A.U.). 



£(rad.). S(deg.). 



•• 

 e. 



|Xred 



9296-0 

 7668-5 

 7596-0 

 6708-2 

 5896-2 

 5350-7 

 4340-7 

 4196-0 

 24960 



•0095 

 •0057 

 •0048 

 •0023 



•o 



-•0015 



0-54 

 0-32 

 0-28 

 0-13 

 0-0 

 — 0-086 



019 

 010 

 •0096 

 ■0040 



•o 



--003 

 -•009 

 -■0096 



-•019 



Li red 



Na(D,) 



Tl green 



1 H violet 



_-0044 — (V9S 



1 



! 



- 1)048 

 - -0095 



-0-275 

 -0-54 



Table III. 



Line. 



Kred 



Li red 

 Na (Dj) 

 Tl green 

 H violet 



X(A.U.). ! £(rad.). 



(fleg.)- 



9296-0 

 7668-5 

 7596-0 

 6708-2 

 5896-2 

 53507 

 4340-7 

 41960 

 2496-0 



•0142 



•0074 



•0071 



•0034 



■0 



- -0023 



-•0064 



-•0071 



-•0142 



0-802 

 0-424 

 0-407 

 195 



o-o 



-0-132 

 -0 367 

 -0-407 



-0-802 



•028 

 •015 

 •014 

 •007 

 •0 



•0046 

 •013 

 •014 

 -•028 



In fig. 5 the values of e given in the above tables are 

 plotted against A., and a comparison of ordinates for given 

 wave-lengths shows the relative efficiency of the three forms 

 for light of that wave-length. The efficiency is unitv for 

 the Di line. Curves I, II, III refer to Tables L, II., III. 

 respectively. When X is > 5896 A.U., e is regarded as 

 positive, and after passing through e = at X,= 5896, € is 

 regarded as negative. Comparing each of the new forms 

 with the Fresnel rhomb, it w T ill be seen that the Bi-rhomb is 

 about twice as efficient as the Fresnel rhomb for any given 

 wave-length, while the Bi-trapezoid has an intermediate 

 efficiency about one and a half times that of a Fresnel rhcmb. 

 But the Bi-rhomb has the disadvantage of being incon- 

 veniently long for a given aperture. This adds to the amount 

 of absorption which, however, is reduced to a minimum 

 b} r the use of uviol glass or fused quartz. In those experi- 

 ments where length is not an objectionable factor, this form 

 may prove useful. In the Bi-trapezoid, in which three 

 reflexions are used, the efficiencv is a little 



ngher than that 



