Norton, Rictor (ed.), Homosexuality in Eighteenth-Century England: A Sourcebook. Updated 7 September 2014 http://rictornorton.co.uk/eighteen/. (Accessed 2014/09/13)
Rictor Norton has assembled an on-line sourcebook of primary documents relating to homosexuality in 18th century England. (He also has several other pages on related historic topics.) He notes: “All the documents faithfully reproduce the spelling, punctuation, capitalization and italicization of the original sources." As is typical for sites covering homosexuality in general, male-related material vastly overwhelms female-related material (which represents less than 10%). I’ve linked and summarized all the items specifically involving lesbians or lesbian-like women, but I encourage interested readers to explore the site on their own.
1708 The Rival Dutchess; or, Court Incendiary Gossip among women regarding a suspicion of “having too great a Regard for my own Sex" and a story of female sexual assault.
1715 The Game at Flats ("Game of flats" is slang for lesbianism. The slang term in this sense can be traced back as early as 1663 though the precise source is unclear, possibly referring to playing cards.) A song by Nicholas Rowe, couching the description in classical and fairly tame imagery.
1720s & earlier Some Cross-Dressing Women Four newspaper accounts of cross-dressing/passing women, though none involve lesbian activity (and two specifically were discovered due to pregnancy).
1728 Two Kissing Girls of Spitalfields A bit of rude doggerel on two women’s sexual activities, providing specifics of what they did with each other.
1732 The Toast Satire on the lesbian "Myra" William King, in revenge against the Duchess of Newburgh to whom he’d lost a lawsuit, wrote a densely allusive satire that portrayed her as promiscuously bisexual and a witch. It is notable for including an early example in English of the use of “lesbian" in its current sense and of describing lesbianism as a sexual orientation with its own cultural traditions.
1734 Pretty Doings in a Protestant Nation A brief disapproving reference to lesbians in Twickenham, framed with classical references.
1739 A Female Dragoon A death notice for a woman who had served as a dragoon in male disguise. No specific lesbian connections are noted.
1744 Travels into Turkey An excerpt from Busbequius’ travelogue discussing lesbian encounters at the public baths, including the unfortunate end of a woman who disguised herself as a man to wed the girl she’d fallen in love with at the baths.
1749 The Game of Flats An excerpt from the polemic “Satan’s Harvest Home" citing Busbequius’ passage above and claiming that the same vice had been taken up by “women of quality".
1755 The Case of Catherine Vizzani Translation of a very long and detailed Italian account of a woman’s adventures in pursuit of the women she loved, both in girlhood and as an adult passing as a man. (Though the translator notes that he declines to include details that “would shock the Delicacy of our Nation". She was twice wounded by rivals, the second time fatally at age 25. Two items make the account noteworthy: the apparent acceptance of her sexual orientation by her parents, and a detailed anatomical examination made of her after death to determine whether there was a physical cause for her orientation (they found none). The translator appended another similar case of a woman courting and marrying another woman while disguised.
1760 Some Cross-Dressing Women Seven assorted newspaper accounts of women passing as men. Only three of them include reference to lesbian-like behavior, specifically marriage to women. In one case, multiple marriages were involved for the purposes of fraud, but in two cases the marriages were of long-standing and in one the wife seems to have been well aware of the truth and interested in continuing the relationship after the public discovery.
1760 A Female Husband Exposed An account of a long-term gender masquerade and marriage to a woman who expressed “strong love and friendship" for her after the disguise was made public and who declined to testify against her in court.
1766 Mary East, the Female Husband Another case of gender disguise and female marriage, but with the motive given as an disinclination of the two women for heterosexual marriage (after unhappy relationships). 34 years later, after the death of the non-disguised partner, the passing woman was unmasked in the midst of blackmail by someone who knew her from childhood. There is no implication of accusation of sexual activity in the record.
1770s-1790s Mrs Piozzi's Reminiscences Diary excerpts from a woman who seems to have had well-developed “gaydar", though only a few of her observations concern lesbians and she has no good opinion of them.
The Macaroni Club: The Adulteress (1773) A satirical poem on homosexuality in both genders, including use of the slang term “Tommies" for lesbians.
1777 Sapphic Love in Portugal A brief reference in a travelogue to the prevalence of “Sapphic love" among the ladies of Portugal.
1778 A Sapphick Epistle A satiric poem transparently referring to the sculptor and actress Anne Damer, who was well known publicly for having female lovers.
1790 Extraordinary Female Affection A newspaper account of the famous “ladies of Llangollen", Elinor Butler and Sarah Ponsonby who eloped and lived together all their lives. Also included is correspondence with a friend of theirs regarding the advisability of suing the newspaper for libel over the article, at which the friend cautioned them to take little notice of it.
1791 Memoirs of Antonina A novel purporting to the be the disguised memoirs of Marie Antoinette, depicting her as a lesbian and having affairs numerous women of the French court (though she is also portrayed as having affairs with a few men).
1821 Cross-Dressing Women in Paris Description of a Polish lady who habitually wears a Polish military uniform, along with a discussion of the open cross-dressing of women in various contexts.