The three spirits all physically embody the ideas they represent. The Ghost of Christmas Past is described as almost ambiguous. It is neither old nor young, neither male nor female. This is because we do not always remember the past accurately, and the past is made up of many parts of us. Conversely, the Ghost of Christmas Past is hardly visible at all. He is not much more than a robe. While the spirit of the past glows and is wistfully vague, the ghost of the future is ominous. We do not know what is in the future, and we are somewhat fearful of it.
The Ghost of Christmas Present goes from young and boisterous to old and feeble. This makes sense, because this is the course of a day. In fact, when Scrooge first meets the ghost and he asks if Scrooge knows his brothers, he says he has not. He then asks how many brothers the spirit has.
“….Have you had many brothers, Spirit?”
“More than eighteen hundred,” said the Ghost. (Stave 3)
This is because the book was written in the 1800’s, so naturally there was a brother for each year. Scrooge realizes that the ghost ages as the day goes on, and at the end he asks him if spirits’ lives are short. This ghost embodies the idea that we only get one present each day, and we need to make the most of it.