In "Merriam-Webster's Dictionary of English Usage" (1989), part of the entry "miniscule, minuscule," notes:
"This spelling [miniscule] was first recorded at the
end of the 19th century (minuscule dates back to
1705), but it did not begin to appear frequently in
edited prose until the 1940s. Its increasingly
common use parallels the increasingly common use of
the word itself, especially as an adjective meaning
`very small.' "
During the last half of the 20th century, dictionary lines have
been adding "miniscule." A telling case comes with the "Concise
Oxford" dictionaries. The Eighth Edition, published in the mid-1980s,
does have an entry for "miniscule," but labels it as "erroneous."
However the "Concise Oxford Dictionary," Ninth Edition (1995) lists
"miniscule" as simply a "variant" spelling.
The "American Heritage Dictionary," Third Edition (1992) gives "miniscule" as a full-fledged variant of "minuscule," as does "Merriam-Webster's Collegiate Dictionary," Tenth Edition (1993). Merriam-Webster's has been listing "miniscule" in their dictionaries since at least 1971.
"The Random House Unabridged Dictionary," Second Edition (1987) lists "miniscule" as a variant, with a usage note stating that while "this newer spelling is criticized by many, it occurs with such frequency in edited writing that some consider it a variant spelling rather than a misspelling."
I am told (on an Internet newsgroup) that "Macquarie's Australian Dictionary," Second Edition lists "miniscule" as a variant spelling as well.
Also noted in the "miniscule, minuscule" entry in "Merriam- Webster's Dictionary of English Usage" is this:
"It may be, in fact, that miniscule is now the more
common form. An article by Michael Kenney in the
Boston Globe on 12 May 1985 noted that miniscule
outnumbered minuscule by three to one in that
newspaper's data base.
That entry concludes with this statement on the spelling
By Cornell Kimball
"Our own view is that any spelling which occurs so
commonly, year after year, in perfectly reputable
and carefully edited books and periodicals must be
regarded as a standard variant."
By Cornell Kimball