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 great 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 re able to hear it. The squeaks 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 Röntgen 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 Röntgen 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 plane, thus opening up before the Ego, as he learns to function through these vehicles, two entirely new and far wider worlds of knowledge and power. Now these new worlds, though they are all around us and freely interpenetrate one another, are not to be thought of as distinct and entirely unconnected in substance, but rather as melting the one into the other, the lowest astral forming a direct series with the highest physical, just as the lowest mental in its turn forms a direct series with the highest astral. We are not called upon in thinking of them to imagine some new and strange kind of matter, but simply to think of the ordinary physical kind as subdivided so very much more finely and vibrating so very much more rapidly as to introduce us to what are practically entirely new conditions and qualities.
It is not then difficult for us to grasp the possibility of a steady and progressive extension of our senses, so that both by sight and by hearing we may be able to appreciate vibrations far higher and far lower than those which are ordinarily recognized. A large section of these additional vibrations will still belong to the physical plane, and will merely enable us to obtain impressions from the etheric part of that plane, which is at present as a closed book to us. Such impressions will still be received through the retina of the eye; of course they will affect its etheric rather than its solid matter, but we may nevertheless regard them as still appealing only to an organ specialized to receive them, and not to the whole surface of the etheric body.
There are some abnormal cases, however, in which other parts of the etheric body respond to these additional vibrations as readily as, or even more readily than, the eye. Such vagaries are explicable in various ways, but principally as effects of some partial astral development, for it will be found that the sensitive parts of the body almost invariably correspond with one or other of the chakrams, or centres of vitality in the astral body. And though, if astral consciousness be not yet developed, these centres may not be available on their own plane, they are still strong enough to stimulate into keener activity the etheric matter which they interpenetrate.
When we come to deal with the astral senses themselves the methods of working are very different. The astral body has not specialized sense-organs — a fact which perhaps needs some explanation, since many students who are trying to comprehend its physiology seem to find it difficult to reconcile with the statements that have been made as to the perfect interpenetration of the physical body by astral matter, the exact correspondence between the two vehicles and the fact that every physical object has necessarily its astral counterpart.
Now all these statements are true, and yet it is quite possible for people who do not normally see astrally to misunderstand them. Every order of physical matter has its corresponding order of astral matter in constant association with it — not to be separated from it except by a very considerable exertion of occult force, and even then only to be held apart from it as long as force is being definitely exerted to that end. But for all that the relation of the astral particles one to another is far looser than is the case with their physical correspondences.
In a bar of iron, for example, we have a mass of physical molecules in the solid condition — that is to say, capable of comparatively little change in their relative positions, though each vibrating with immense rapidity in its own sphere. The astral counterpart of this consists of what we often call solid astral matter — that is, matter of the lowest and densest sub-plane of the astral; but nevertheless its particles are constantly and rapidly changing their relative positions, moving among one another as easily as those of a liquid on the physical plane might do. So that there is no permanent association between any one physical particle and that amount of astral matter which happens at any given moment to be acting as its counter part.
This is equally true with respect to the astral body of man, which for our purpose at the moment we may regard as consisting of two parts — the denser aggregation which occupies the exact position of the physical body, and the cloud of rarer astral matter which surrounds that aggregation. In both these parts, and between them both, there is going on at every moment of time the rapid inter-circulation of the particles which has been described, so that as one watches the movement of the molecules in the astral body one is reminded of the appearance of those in fiercely boiling water.
This being so, it will be readily understood that though any given organ of the physical body must always have as its counterpart a certain amount of astral matter, it does not retain the same particles for more than a few seconds at a time, and consequently there is nothing corresponding to the specialization of physical nerve-matter into optic or auditory nerves, and so on. So that though the physical eye or ear has undoubtedly always its counterpart of astral matter, that particular fragment of astral matter is no more ( and no less) capable of responding to the vibrations which produce astral sight or astral hearing than any other part of the vehicle.
It must never be forgotten that though we constantly have to speak of "astral sight" or "astral hearing" in order to make ourselves intelligible, all that we mean by those expressions is the faculty of responding to such vibrations as convey to the man's consciousness, when he is functioning in his astral body, information of the same character as that conveyed to him by his eyes and ears while he is in the