Yet we need only imagine that a large glass plate is put in the place of the curtain covering the whole stage.

Now we see the stage through the glass; and if we look at it with one eye only it is evident that every single spot on the stage must throw its light to our eye by light rays which cross the glass plate at a particular point.

For our seeing it would make no difference whether the stage is actually behind that glass plate or whether all the light rays which pass through the plate come from the plate itself.

If those rays with all their different shades of light and dark started from the surface of the glass plate, the effect on the one eye would necessarily be the same as if they originated at different distances behind the glass.

This is exactly the case of the screen.