might refer to the subject matter of the pictures, Line.,

Ellizabeth might be familiar with Gilpin's works, and, indeed, he teaches you how to look at things- perspective, what constitues a good grouping of objects etc etc. He does not teach about allegorical paitings, such as those executed by old masters such as Nicolas Poussin.

I tend to think the Pemerley art collection, like its library, is the work of many genrations and therefore will posess many olde,r allegorical paintinngd which refer to classical mythology.