different between dulness vs dullness

English

dulness (usually uncountable, plural dulnesses)

Obsolete spelling of dullness
- 1843, Thomas Carlyle, Past and Present, book 3, chapter XIII, Democracy
  A smack of all Human Life lies in the Tailor: its wild struggles towards beauty, dignity, freedom, victory; and how, hemmed in by Sedan and Huddersfield, by Nescience, Dulness, Prurience, and other sad necessities and laws of Nature, it has attained just to this: Grey Savagery of Three Sacks with a hem!

From dull +? -ness.

dullness (usually uncountable, plural dullnesses)

The quality of being slow of understanding things; stupidity.
The quality of being uninteresting; boring or irksome.
Lack of interest or excitement.
The lack of visual brilliance; want of sheen.
(of an edge) bluntness.
The quality of not perceiving or kenning things distinctly.
(archaic) Drowsiness.
- c. 1610-11, William Shakespeare, The Tempest, Act I scene ii^[1]:
  Prospero: […] Thou art inclin'd to sleep. 'Tis a good dulness, / And give it way— I know thou canst not choose.

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