It is important to note in Fig. 3B that the primary focallengthffor the convex
surface [diagram (a)] is not equal to the secondary focallengthj' of the same surface
[diagram (c)]. It will be shown in Sec. 3.4 that the ratio of the focal lengths f'Lf is
equal to the ratio n'/n of the corresponding refractive indices [see Eq. (3e)]: