126 Dr Searle, The determination of the focal length, ete. 
the principal point, are both in front of the lens. The focus of the 
lens remote from the mirror is one point, and here an inverted — 
image is obtained. This point is therefore the “centre” of the 
; nite nO Sai 20 
system. The other point is that point which has for its conjugate — 
with respect to the lens the point where the axis of the system 
cuts the plane mirror. Here the image is erect. This point is 
therefore the principal point*. But, since the image is erect, it is 
hidden by the object when the adjustment is complete, and this 
furnishes a test of the adjustment. If a well-polished needle be 
used and a light from a flame be concentrated upon it by means 
of a large lens, a halo of coloured light will be seen round the pin, 
the colour being due to chromatic aberration and changing with 
the position of the pin. The light should fall as much as possible | 
on the back of the needle. 
Since the “centre” of the system is nearer to the lens than — 
the principal plane of the system, the system behaves like an 
ideal convew mirror. 
* Mr R. HE. Baynes has pointed out to me that the position of the principal 
plane of this system, or of any more general system, is independent of the curvature 
of the surface M (Fig. 1), at which the reflexion occurs, provided that the vertex of 
that surface is always in the same position. 
