I’ve just worked this out. It’s like he’s looking at this skull in the gravedigger’s hands and he just sees a skull – but then when he looks (or touches) it, he can suddenly see the skull as the structure below the face of his childhood companion. When he sees it, it touches him and so he feels compassion and pity for his long gone friend/father-figure/loved one. He holds the double-ness of the dead object in his hands and the memory of the live person it was once a part of.