Long versus short focal length refractors

I recall reading one time that since refractors
effectively spread out the light spectrum, the
shorter the focal length, the greater the spread.

A formula was given that to keep the effective
"spread" from becoming too great (more than
one wave of light), the focal length had to be
three times the diameter of the objective as
measured in inches times the diameter of the
objective. Thus, a refractor with a focal length
of three inches (76.2mm) needed to be 3x3 or
F9. Or 27 inches (686mm) in order to hold the
spread of the light spectrum below one wave.

How accurate is this? Anyone know more?

Jerome Bigge
Member, Muskegon Astronomical Society
Author of the "Warlady" & "Wartime" series.
Download at "http://members.tripod.com/~jbigge"