Orthosia incerta may be complete or broken up into three blotches; upper stigmata large, the reniform generally dark, both edged with pale; a strong thick median shade, sometimes bent at middle and entire, often marked only at costa and inner margin; hindwing pale or dark grey with dark cellspot and pale fringe. The markings are all clearer in the grey unsuffused forms: of these the chief are pallida Lampa from Sweden with the ground colour pale grey, finely striated with dark, and the two dotted cross lines;- "coerulescens" Tutt with the ground colour pale bluish grey; — "subsetaceus" Haw.which is dark slaty
The Missing Shade of Blue he later says, "this instance is so singular, that it is scarcely worth our observing" he cannot be referring to this particular example, but rather to the "type" of exception that it represents. It is also said that when Hume says, "Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain that he will perceive a blank, where that shade is wanting", he is assuming that colours are composed of a set of distinct independent hues, when in reality they form a continuum.