The microscope shows them to us.

Now why should not the air, inaccessible to the sight and to the instruments of man, swarm, like the other elements, with beings more or less corporeal, embryos more or less mature ?" "That is probably why cats suddenly look upward and gaze curiously into space at something that is passing and that we can't see," said the bell-ringer's wife.
"No, thanks," said Gevingey to Des Hermies, who was offering him another helping of egg-and-dandelion salad.
"My friends," said the bell-ringer, "you forget only one doctrine, that of the Church, which attributes all these inexplicable phenomena to Satan.

Catholicism has known them for a long time.

It did not need to wait for the first manifestations of the spirits--which were produced, I believe, in 1847, in the United States, through the Fox family--before decreeing that spirit rapping came from the Devil.

You will find in Saint Augustine the proof, for he had to send a priest to put an end to noises and overturning of objects and furniture, in the diocese of Hippo, analogous to those which Spiritism points out.