W. Harkness—Color Correction of Achromatic Telescopes. 195 
the extremities of these lines a closed curve must be drawn. © 
The figure thus obtained will be termed the illumination dia- 
ram, because it exhibits the amount and distribution of the 
e.), of the diagram. Hence, to find the position of the 
ocal plane, we have only to cut out the diagram (which should 
the same telescope when used for different purposes. 
example, if it were required to find the interval between the — 
is proportional to the distance between that plane and the febt 
of the focal curve corresponding to the wave length o the ight. 
me | 
snl ally —n') — Ge — eb T nee 
in which 7,, is the reciprocal of the wave length corresponding 
to either of the two points in which the focal plane cuts the 
