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



which is one of the largest and most recently constructed 

 instruments of this class, we have for b. (f>, "S, and n the 

 following values* : — 



5=5*1 cms. 



<£ = 63° 35' (mean value). 

 N=3. 



n — ^l-(553 for \=5500f 

 j 1-678 „ \ = 4300 

 1 1-693 „ A = 3900 f. 

 Introducing these values and solving for Bb we find 



Bb = /3x rl6-4t for X=5500 

 \ 17-2 ., \=4300 



( 



17-8 „ X=39O0 



The absorption of the glass of which this prism-train is 

 composed (0*102) has been determined by Yogel and Mullert. 

 The values of j3 computed from their results are as follows: — 



For wave-length 5500, /3=*021 

 4300, /3=*071 

 „ 3900, £='37 



For the visual region of the spectrum, therefore, the value 

 of Bb for this spectroscope is about *34, and the corresponding 

 value of or, as given by (18), is less than 1*003 a , i. e. the 

 resolving-power is diminished less than one-third of one per 

 cent, by the result of absorption. 



In the photographic region of the spectrum, however, the 

 case is quite different. At wave-length A, =4300, which is 

 about the middle of the photographic region, the value of 

 Bb for the Bruce spectroscope is 17*2 X '071 =1*22, and the 

 corresponding value of cr is 1*04, i. e. the resolving-power at 

 this point in the spectrum is reduced about four per cent, by 

 the result of absorption. At wave-length \=3900 the value 

 of Bb is 6*59, the value of a is 2*10 a d , and the resolving- 

 power is therefore diminished by more than fifty per cent, 



For prism spectroscopes of larger size or greater resolving- 

 power than the one above considered the effect of absorption 

 may become serious, even in the visible part of the spectrum, 



* •' The Bruce Spectroscope of the Yerkes Observatory," E. B. Frost, 

 Astrophysical Journal, vol. xv. pp. 12-17, Jan. 1902. 



f Estimated by exterpolation from the values given by Frost. 

 X Astrophysical Journal, vol. v. p. 82 (1897). 



