Hi Prue,
I have just come across this thread.
Looking at your picture, it seems to me EITHER, that it cannot be of the man you think it is OR, that your 1780's date is not correct.
Assuming the man in the picture to be between 40 and 50 years of age, then, judging by his costume, hair, clean-shaven upper lip, and beard ---- my guess is that the earliest he was born was about 1815 to 1820.
He does give the impression of being a clergyman. Too emaciated to have been an industrialist or businessman of the period : and too other-worldly looking to be a lawyer.
Early photographs almost as a rule feature one or more books -- intended to show that the subject was literate. HOWEVER ! Engravings , particularly portrait engravings mounted in books, did not do so for the obvious reason that the subject invariably was literate.
In your instance, I think the inclusion of the book indicates a connection between the man and the book --- or books in general. An Author, Poet, or Clergyman perhaps ?
PERCY