206 Dr Searle, Experiments illustrating f 
r 
reflected at the cemented surfaces is inappreciable, and for the 
present purpose such a cemented component of a lens system is” 
to be considered as a single lens. It is only the air-glass surfaces 
which count. 
The image of any object formed by rays which have not) 
suffered reflexion in their passage through the lens will be called 
the primary image of that object, and any image formed by rays 
which have suffered two reflexions will be called a secondary image 
(Nebenbild) of the object. 
§ 2. Ghosts in photography. In photography the sensitive 
plate is adjusted so that the primary image of an object, ie. the 
image formed by rays which have not been reflected im their 
passage through the lens system, is sharply focussed upon it, | 
Thus, if S (Fig. 1)* be an object point on the axis of the lens. 
A 
VLLLIZZ 
i 
SW 
Fig. 1. 
system, the plate is placed in the plane GH which is normal to 
the axis and passes through the conjugate primary image point 7, 
If 7, be a real or virtual secondary image of S formed by twice 
reflected rays, the cone of rays which meet in 7, will be cut by 
the plane through 7’ in a circle of diameter GH. If T, is so far 
from 7’ that the diameter of this circle is greater than that of the 
sensitive plate, the intensity of illumination due to 7, will be very 
small and the only effect will be a slight uniform fogging of the 
plate. The effect is so small that the number of air-glass | 
surfaces has to be considerable before there is any serious fogging — 
due to all the cones of twice reflected light whose sections by the 
plane GH have diameters greater than that of the plate. | 
If, however, 7’, be close to J, the illuminated patch GH will 
be small and the illumination may then be sufficient to cause 
trouble. This small patch is known as a “ghost.” 
If a secondary image 7’, coincides with 7, any object in a plane 
normal to the axis and passing through S will have its corre- 
sponding secondary image in the plane GH. But the secondary 
s) 
Be at RAIL LR A OE I Ry ig SE 
* Fig. 1 is merely diagrammatic and does not show accurately the paths of 
the rays. 
