CASSEGRAIN REFLECTOR WITH CORRECTED FIELD. 37 



the third lens the original lens with the surfaces in reversed order. This reversal 

 of order will replace B, B' respectively l>y B', B. Hence k, ) will equal k", J>" 

 respectively, but q + q" = 0. 



in the ions (9), using ' to denote the mirror surface 



. (11) 



Thr same expression is true of a more complicated reverser of any numl>er of thin 

 lenses with the last surface silvered. Also 



(12) 



To conclude this preliminary discussion of systems of thin lenses in contact I shall 

 introduce a system which consists of two thin lenses in contact, of equal and opposite 

 focal length and of the same glass, and therefore a null system in every respect 

 except for aberrations. The use of such a system will l>e illustrated hereafter. Its 

 simplicity is such that its aberration-coefficients reduce to very easy forms, and can 

 therefore be handled algebraically in an experimental investigation, in order to 

 discover what system will correct the aberrations of a proposed system ; it will 

 supply a useful approximation to a solution when any less idealised system is too 

 complicated to manage. 



From the expressions (8) we have for the Thin Corrector 



K = k + k' = 0, $ = kn + kn = 0, 



k (k-k')k'n 



..... (13) 



and all the rest of the coefficients run in agreement with p. 35, so that 



