Another fact which needs to be considered in this connection is that different human beings vary considerably, though within relatively narrow limits, in their capacity of response even to the very few vibrations which are within reach of our physical senses. I am not referring to the keenness of sight or of hearing that enables one man to see a fainter object or hear a slighter sound than another; it is not in the least a question of strength of vision, but of extent of susceptibility. For example, if anyone will take a good bisulphide of carbon prism, and by its means throw a clear spectrum on a sheet of white paper, and then get a number of people to mark upon the paper the extreme limits of the spectrum as it appears to them, he is fairly certain to find that their powers of vision differ appreciably. Some will see the violet extending much farther than the majority do; others will perhaps see rather less violet than most, while gaining a corresponding extension of vision at the red end. Some few there will perhaps be who can see farther than ordinary at both ends, and these will almost certainly be what we call sensitive people--susceptible in fact to a greater range of vibrations than are most men of the present day. In hearing, the same difference can be tested by taking some sound which is just not too high to be audible--on the very verge 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.