Mrs. Clennam's Watch
When Arthur comes back from China, he brings his mom his dead father's watch, which has the inscription D.N.F. engraved on it, which stands for "Do Not Forget." That's all well and good, but this phrase is so cryptic and hard to interpret that whatever Arthur's father might have meant is lost in translation. Which sort of raises the question – a watch with initials? Really? Couldn't old Mr. Clennam have just written a letter?
In any case, the whole Mrs. Clennam subplot rests on how she's supposed to read the phrase "Do Not Forget." Hint: she doesn't get it right. For her, a woman who's been convinced all her life that she is some sort of God's vengeance on earth, "Do Not Forget" means "don't forget all those horrible sins that you're supposed to keep on avenging till you die." And she doesn't. She never forgives Arthur's father, never loves Arthur or treats him as a son, and never tries to make amends for keeping the Dorrit family needlessly locked away in prison. For her, the fact that she is in a wheelchair makes it OK for the Dorrits to rot in the Marshalsea.
Sadly, though, what Arthur's dad meant by "Do Not Forget" was "don't forget to make some restitution for all the evil you've done, which I have gone along with." These two interpretations are at the heart of Mrs. Clennam's total misunderstanding of Christianity as Dickens saw it: instead of a religion of mercy, love, and forgiveness, she sees only punishment, vengeance, and eye-for-an-eye justice.
The Shadow of the Marshalsea
At first the shadow of the Marshalsea is a real thing: the high walls of the prison block most light from coming into the little yard. But soon, the idea of a Marshalsea shadow falling on someone comes to mean the long-term effects of having been a prisoner there. Whenever Dorrit starts to really act up, Amy thinks she can see the shadow of the wall on him. Even more sadly, when Amy has a hard time adjusting to the rich lifestyle the Dorrits lead in the second half of the novel, she says she herself can never get out from under that shadow. It's a strong and moving image – easy to imagine visually and so full of rich meanings. Think about it – what kind of things do we usually associate with shadows? Are there positive meanings as well as negative ones? Who else does the Marshalsea shadow touch?
For a little while, Arthur splits into two personalities. There's regular old Arthur, a practical, quiet, tired, generally low-energy man. And then there's Nobody, a guy who suddenly finds himself in love with the young Pet Meagles and starting to have fantasies of a future together with her. Why do we get this odd way of showing how Arthur is trying not to fall for this girl? It might be a handy visual for demonstrating sexual repression and just how messed up Arthur feels for wanting to be with this young woman. It's also a neat parallel to Pet's dead twin – another Nobody who, under different circumstances, might have ended up Arthur's wife. (Let's not get too hung up on the fact that this means Arthur is really only in there for Pet's looks – anyone who looks like her would apparently do.) When Arthur finally gives up this aspect of his personality – youth, desire, hope for a sexual future – he sends a rose floating down the river near Meagles's house. Good-bye, Rose. Good-bye hopeful Arthur. Immediately afterwards, he starts thinking of himself as an old man.