erge of
audibility as it were--and discovering how many among a given number
of people are able to hear it. The squeak of a bat is a familiar
instance of such a sound, and experiment will show that on a summer
evening, when the whole air is full of the shrill, needle-like cries
of these little animals, quite a large number of men will be
absolutely unconscious of them, and unable to hear anything at all.

Now these examples clearly show that there is no hard-and-fast limit
to man's power of response to either etheric or aerial vibrations, but
that some among us already have that power to a wider extent than
others; and it will even be found that the same man's capacity varies
on different occasions. It is therefore not difficult for us to
imagine that it might be possible for a man to develop this power, and
thus in time to learn to see much that is invisible to his fellow-men,
and hear much that is inaudible to them, since we know perfectly well
that enormous numbers of these additional vibrations do exist, and are
simply, as it were, awaiting recognition.

The experiments with the Roentgen rays give us an example of the
startling results which are produced when even a very few of these
additional vibrations are brought within human ken, and the
transparency to these rays of many substances hitherto considered
opaque at once shows us one way at least in which we may explain such
elementary clairvoyance as is involved in reading a letter inside a
closed box, or describing those present in an adjoining apartment. To
learn to see by means of the Roentgen rays in addition to those
ordinarily employed would be quite sufficient to enable anyone to
perform a feat of magic of this order.

So far we have thought only of an extension of the purely physical
senses of man; and when we remember that a man's etheric body is in
reality merely the finer part of his physical frame, and that
therefore all his sense organs contain a large amount of etheric
matter of various degrees of density, the capacities of which are
still practically latent in most of us, we shall see that even if we
confine ourselves to this line of development alone there are enormous
possibilities of all kinds already opening out before us.

But besides and beyond all this we know that man possesses an astral
and a mental body, each of which can in process of time be aroused
into activity, and will respond in turn to the vibrations of the
matter of its own p