216 Dr Searle, Experiments illustrating 
[DC] Foo =1/foo=M +N, 
[DB] Fozp=1/fpp=M + N.+ W 
[DA] Fo4=1/for= M+ N2+ W 
[CB] Fong =1/fop =$M +N 4+W i 
[CA] Fo, =1/fo4=M2+N + W 
[BA] Foz=1/fe,= M+ N. 
If the lenses are such that each of the six secondary powers of 
the system is positive, it will be possible to obtain six real secondary 
images of a real object. 
A convenient system is obtained by using two meniscus lenses ~ 
of positive powers, and placing them in contact so that their 
concave surfaces face each other. Then M, N, M,, N, are positive 
and, since b and ¢ are negative, W is positive. To avoid coin- 
cident images, M and WN should be unequal. Meniscus lenses are 
used in spectacles, under the name of “ periscopic” lenses. 
§ 12. Haperimental details. The six secondary images are 
readily observed if the system consists of two positive meniscus 
spectacle lenses (price 9d. each) placed in contact, with their 
concave surfaces facing each other. The following description 
refers to this case. 
The primary and secondary focal lengths of each lens are 
found just as in § 9. 
The radius of curvature of the concave surface of each lens is 
found by making a pin coincide with its image formed by reflexion 
at that surface. The convex surface is smeared with vaseline to 
stop reflexion there. The common distance of the pin and its 
image from the surface is equal to the radius. Since the surface 
is concave, a square-ended scale cannot be used to measure the 
distance; a proper appliance must be used. 
The two lenses are then mounted on the tube as in Fig. 4. 
When S is sufficiently far from the lenses, the six real secondary 
images of S will be easily seen on looking through the system 
towards S, provided the eye be far enough from the lenses. If 
the images are very small, they may be increased in size by 
bringing the lenses nearer to S. If, however; the distance is 
made too small, some or all of the six secondary images will 
become virtual. 
The six secondary focal lengths are best found by the minimum 
distance method. It is best to begin with the one of shortest 
focal length. 
It is impossible to tell by the appearance of any particular 
secondary image which two surfaces have acted as reflectors for 
that image, as long as the lenses are fiwed together. But if we 
