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



To show the advantage of the lighter glasses on the score 

 of photographic purity the values of 11,3 have also been com- 

 puted for the dense flint, 0*93, and the light flint, 0'340, of 



Schott's table. The constants n, ft, and ~ for wave-length 



3900 for these 1 two glasses are as follows : — ■ 



0-93 %9oo=l-66; ^=3500; ft^'U 

 0-340 ^900=1*61; ^ =3100; ft^'08. 



The values of Ktf for these values of ^r and ft are tabulated 



dX 



in columns III. and IV. of Table II., and plotted as curves 



II. and III. of fig. 3 (PL VIII.) . The points of maximum 



for these curves jire found at 



r m ri 315000 for glass 0'93, 

 r m ^ 488000 „ „ 0-340. 

 and the corresponding values of Us (max.) are 

 157000 for 0-93, 

 244000 for 0-340. 



As in the preceding case the form of the curves is such that 

 it is not desirable to go to the extreme maximum values of 

 of R^, but to stop at points corresponding to r ^-200000 



and 2:350000 respectively. 



For the visible portion of the spectrum the values of 



>/, ttt, and ft for the first glass considered. 0*102, are as 



follows: — 



n= 1-G53; f-'—lloO, /3='021. 



.-. ft dX =-000018; 

 an 



r m ^ 740000; 



Up (max.) ZT 370000. 



The above results show that while the Bruce spectroscope 

 is quite efficient for the visible part of the spectrum, it is 

 quite otherwise for the photographic region in the neigh- 

 bourhood of the H and K lines. Such an instrument would 

 not be well suited, therefore, for solar work, particularly of a 

 spectroheliographic nature. 



