FOCUS AND OPTICS 39 
side as before. Produce these two lines so that they meet 
in the point A, Fig. 2. Bisect the angle BAC by the line AD, 
5 B 
y Cc 
Fig. 2 
and draw a line EF at right angles to this line, equal to the 
separation of the pencil marks on the ground glass; the focal 
length is then AG. 
Smith's Method—T. Smith, of the National Physical 
Laboratory, London, has given another method of finding 
the equivalent focus of a lens, based on the focussing of the 
image of a distant object on the lens axis upon the ground 
glass of the camera, using first the complete lens and then 
each component separately. The optical rule that serves as a 
basis of the method is that the focus of a lens of focal length f 
is at a distance equal to fF ~ f’ from that of the combination 
of focal length F formed by placing in front of the first lens 
another of focal length f’. The method is carried out as 
follows: fit the complete lens to the camera, and focus sharply 
on a distant object. Mark the position of some part of the 
lens front or moving baseboard against a fixed part of the 
camera. Now remove the front combination, and again focus 
on the object, noting the distance d through which the lens 
front requires to be racked out; this distance is fF +f’ in 
which F is the focal length of the whole lens and f and f’ 
those of the combinations. Now focus again with the whole 
