The Patersons rely on your focusing on an aerial image. That's why you adjust for the line to be sharp.
The grain isn't on a surface but is supposed to be seen at the same plane as the line.
I've never been able to use one myself, I can't seem to wrap my mind(eyes) around how it works. I end up using the image on the base board & reading glasses. A large diameter magnifying glass might also work.