Thanks to Google Books, I've recently discovered an essay by Lady Mary Shepherd that is not in the Thoemmes Press edition of her works:
On the Causes of Single and Erect Vision
In Essays on the Perception of an External Universe there is another essay on this topic (Essay XIV), "On the Reason Why Objects Appear Single Although Painted on Two Retinas, and Why They Appear Erect Although the Images Be Inverted on Them." The above essay, however, is a different essay, and postdates the Essays, as can be seen from the citations in the text and from the fact that it was published in The Philosophical Magazine in June 1828, the year after the publication of the Essays. This essay was therefore never published in book form, which is no doubt why it was missed. Another project I'm going to have to undertake at some point: compare the position taken in this essay with that taken in Essay XIV.
Any future edition of Shepherd's works, then, will need to include not merely the works in the Thoemmes Press edition, but also this essay and the letter preserved by Blakey.