206 Dr Searle, The determination of the effective aperture, etc. — 
infinite distance, and if the stop marked //n is to be used when 
the surface is at a fimite distance, then nt =n,?t,, where t/t, 1s_ 
given by (14) or (15). Hence 
ei Te 1 
n=n a 
rain = Ne - 1 + me" = 4.06) 
pb) 7? + OF 
§ 10. Practical example. The same system was used as — 
in § 8. 
The distance from F, to the plane of the end of the lens mount was 
16:16 cm. The distance x between that plane and OQ, was found in § 8 to be 
3:42 cm. Hence 0, F,=16:16+3°42=19°58 cm. 
The focal length f was found in § 8 to be 21°59 cm. 
Hence B=0,F, /f=19°58/ 21:59 =0:907. 
The exposure for a given stop can then be calculated by (15) and the stop — 
for a given exposure can be found by (16). 
