Astonishing Stereoscope by Erik Blegvad, Jane Langton (1983, Paperback)

Five mysterious cards ...

When Eddy Hall receives five cards for his stereoscope, he and his sister, Eleanor, can't wait to see what exotic places they reveal-maybe Stonehenge, or a centuries-old European cathedral. But instead, when they look through the stereoscope, Eddy and Eleanor see some very strange things. An odd-looking rope hangs from the sky down into every picture. A marmalade-colored cat that looks suspiciously like Herm, the family cat, also appears. And one picture looks like the front hall of their very own house! The images seem to be almost real, not just three-dimensional illusions. All it will take is one little tug on that rope to find out for sure....