This property is referred to as the "irrationality of dispersion."
The general test of irrationality which he established is that, if a l a, a, be an interminate continued fraction, a l, a2,
Moreover the greater depths of the curves (or "curvature powers") in itself neutralize more or less the advantages obtained from the reduced irrationality of dispersion.
This want of proportion in the dispersion for different regions of the spectrum is called the "irrationality of dispersion"; and it is as a direct consequence of this irrationality, that there exists a secondary spectrum or residual colour dispersion, showing itself at the focus of all such telescopes, and roughly in proportion to their size.
These glasses, however, still hold the field, although glasses are now produced whose irrationality of dispersion has been reduced to a very slight amount.