CASSEGRAIN REFLECTOi; WITH CORRECTED FIELD. 4fl 



even suffixes, thus t a = O S O'., = < >'J >" .... /, = <>,O' 4 , / = OgO',, ; for the separations, 

 d, = 0,, d, = 0",0 4 , d 6 = O'A, 



H,, H" a are the unit points of the reverser ; H 4 , H' 4 and H a , H',,, those of the two 

 lenses of the corrector. Similarly F, is the principal focus for the great mirror, 

 F 3 , F' 3) F" 3 for tin- ditferent surfaces of the reverser, and so on, the final focus of the 

 whole combination bein<j F" 7 . 



Writing, as above, 



* = (l-i)(B-B'), 7 = (l + i)(B + B'), q = 9/K, 

 we find by considering the scheme 



, -in, or ,, i r. H- 



.(n-l)B, nj L* U Un^-lJF, n-'J [K, L. 

 that for any thick lens 



K = K -n(l-n- 1 



and 



. (24) 



For the reverser we have the scheme, including reversal of the ray at the reflection, 



i-i)B,, Jl* ij l.2B' z , ij 1* ij -(W-'-OB,, w-'J [K,, L, 



whence 



K 2 = 2n 



= -O",H" a ............. (25) 



Write (K a ) for the part of K 2 which is due to the lens of the reverser, namely, 

 (K,) = (l-- 1 



By methods essentially the same as those exposed below I was led to the following 

 approximate values as a system corrected for aberrations : 



B =- '250000, e u = +'16502, a = +'200000, 



d, = +1 '320 133, 



B a = -B" 2 = +'469009, B' a = +'450653, t t = +'020000, 



d 3 = +'906760, 



B 4 = -'697845, B' 4 = +2'043309, t t = +'012500, 



c? 5 = +'002500, 



Bj = +'003705, B' 6 = -2'610677, t t = +'012500. 

 VOL. ccxiir. A. H 



