Julia rocks! She really isn't afraid of anything, even if she's taking some incredible risks. I can't think of another TV character like her, then or now.
I think Julia is working from the evidence that she herself has collected. Even without Willie's blood sample, the mirror test would prove her suspicions beyond a doubt.
David is scared when Vicki goes into Josette mode while listening to the music box. And we learn that he was named after an earlier David Collins, but since the earlier David is never mentioned again, we don't need to worry about him. Our David tells Julia about his friend Sarah, who looks just like the picture in the family history. Julia is immediately alert when David mentions that he's only ever seen Sarah near the Old House.
Vicki tells Julia about the maniac who kidnapped Maggie Evans. But you wouldn't know about that, she adds, and Julia has to play along. She's even more interested when Vicki says that the only mirror in the Old House is in Josette's room.
Burke really is turning into a first-class tyrant. We can at least hope that the writers have done this on purpose, but it's awfully heavy handed. But I suppose that in the mid-1960s, that was the best they could come up with for a plot twist that would make Vicki turn more and more to Barnabas.
Julia once again boldly goes to the Old House, makes herself comfortable and shows Barnabas the portraits. When she gets to Naomi, Barnabas can't resist taking the book himself, though he turns away from her. She encourages him to turn the page to see the portrait of Sarah, the “first” Barnabas’s sister. As she natters on about how much Sarah’s eyes look like his, in her riskiest move, she surreptitiously takes out her compact, holds the mirror at the proper angle, and sees-- exactly what she expected to see.
Barnabas angrily asks, What are you doing? But she pleads feminine vanity--even historians have to check their makeup. Even in the face of Barnabas's fury, she is utterly unruffled and tells him, I hope we can talk again. I love her self-confidence when she tells Vicki that she's learned everything she needs to know.