Perhaps the most labor-intensive part of the making of the Dictionary was the process of finding quotations from literature to illustrate the use of each word. Johnson was very particular in his choice of writers to quote from; he refused to cite those, such as Hobbes, whose prose he admired but whose principles he rejected. Quite beyond reproach, of course, was Virgil, and this 1740 translation by Christopher Pitt is one of two books in the Hyde Collection marked for use in the Dictionary (another 11 survive in other libraries).
I apologize for the faintness of the marks in this scan. I tweaked the settings to bring out the light pencil marks as much as possible. I hope you will be able to make out Johnson’s system, which involves underlining the word in question, noting its first letter in the margin, and bracketing the passage to be quoted. Apparently this book fared much better than some which Johnson employed; he was notorious for returning borrowed books to his friends so heavily marked in dark pencil as to be unreadable.