George Myers wrote:

>I could not find any reference here but a suggested source from the above
introduction may be "The Classical Dictionary of the Vulgar Tongue," 1785
by
Captain Francis Grose.<

The 1811 edition of this masterpiece has been reprinted, as 

Grose, Francis, "1811 Dictionary of the Vulgar Tongue: A Dictionary of
Buckish Slang, University Wit, and Pickpocket Eloquence."  Digest Books,
Northfield IL, 1971 (Unabridged reprint of 1811 edition).

It is unpaginated, but the entry for looking glass reads:

"LOOKING-GLASS.  A Chamber pot, jordan, or member mug."

LJ Cook