two luminous undulations thus related. In the same way one sound-pulse may obliterate another, and two sounds thus result in silence. If, for example, a vibrating tuning-fork, placed near the ear, be slowly turned on its axis, four times in each revolution, a position will be reached where no sound is heard. This occurs when the prongs are oblique in reference to the ear, and the waves generated by each prong exactly interfere. If the interference be only partial, beats, or intervals of sound and silence, will be produced.

Returning now to the sounding-bell: its motion chiefly spends itself on the air. Hence, when we remove the air from around a sounding body, the vibration continues far longer than otherwise it would. In such a case no sound is heard: internal friction finally brings the sonorous body to rest as a whole, and molecular motion or heat is the result. But in removing the air we do not, and we cannot in any way as yet known, remove the luminiferous ether; by which ether heat is propagated as well as light. Hence the heat-motion resulting from what originally was a sounding body at last escapes from the enclosure in the form of ethereal undulations.

Now when heat spreads by the ether in this way, we denominate it radiant heat; and, as we might expect, all the laws common to light are also common to radiant heat, for both are undulatory motions of the selfsame ether. But whilst the propagation of light only takes place (so far as we yet know) by means of this ether, heat, on the contrary, spreads itself in two ways. It may be propagated from particle to particle by tangible matter, and the phenomenon is then termed conduction; or in waves by the ether, and the phenomenon is then, as we have seen, termed radiant heat. Now sound, like heat, may also be propagated by gross matter, and this likewise is called conduction; it may further, as already mentioned, spread itself in waves by the air, and in this form we may conveniently call it radiant sound. Light being only known in the radiant form, obviously it will only be in the phenomena of radiant sound that we shall be justified in further seeking the analogy to light. This we shall do in a succeeding section.

Here, however, it is important to note that we might expect an analogy between the phenomena of sound-conduction and heatconduction. This is the case: taken in connection with the footnote below, considerable support is thus given to the analogy of sound and light. Broadly viewed, the best conductors of sound are the best conductors of heat, and vice versa. Conduction in both cases is best through solids and worst through gases. Metals are the best conductors of sound and also of heat. In one very

The selective absorption of radiant heat by various media (thermocrosis), the phenomena of calorescence-or the conversion of radiant heat into light — and the warming of a black surface by purely luminous rays, demonstrate the fact that radiant heat is only another phase of light.

curious and well-established instance (though probably by no means the only one), the analogy runs very close. It has been observed that in wood sound is conducted with different facility in three directions. Lengthways, or along the grain, the conduction is best; across the rings or grain it is much worse; and tangential to the rings it is worst of all. Exactly the same facts, and in the same order, have been found to hold good as regards the conduction of heat in wood. But what is true of the conduction of heat is in every case, even in this last, equally true of the conduction of another force, namely, electricity. Now the conduction of electricity strongly resembles the conduction of sound; inasmuch that while the rate of propagation of sound through liquids is increased by increase of temperature, it is decreased in solids by the same cause. So likewise it is found that the same causes produce the same effects on similar substances in the rate of propagation of electricity. Thus, in the first place, we link light on to heat, and then heat on to sound; after that, heat on to electricity, and here, at last, electricity on to sound, whilst the connection between sound and light it is our object to set forth in this article. How this incidental fact opens up for a moment the oneness of the diverse forces which play around the world! We are encircled with wonder and with mystery, but every now and then facts such as these arise, which lead us to believe that perfect unity and simplicity lie somewhere in the back-ground.

§ 3. THE PERCEPTION OF LIGHT AND SOUND.

At first it would appear hopeless to seek for any analogy between organs so essentially different as the eye and ear. It must, however, be borne in mind that the functions of those organs are not only to receive the impressions of light and sound, but also to gather up and suitably present the wave-motion which impinges upon them. Owing to the vast difference in tenuity and elasticity of the media which convey light and sound, we should expect to find the very difference we observe in the apparatus contrived for hearing and seeing. Nevertheless it is possible to trace some correspondence of parts in the eye and the ear; and there have not been wanting physiologists who have pushed this view to a detailed and fanciful extreme. The reader who wishes for further information on this presumed resemblance of the two organs will find it given in a recent work by Dr. Macdonald, to which we may have occasion again to refer.

But there is a remarkable analogy, very much overlooked, which appears to hold good between the perception of the respective impressions of light and sound. When we consider the multitude and complexity of the sounds we hear in a concert, and when we remember the extent, the diversity of colour and appearSound and Colour,' by Dr. Macdonald, F.R.S. Longmans, 1869.

ance of a landscape, it would appear almost inconceivable how such varied impressions could be individually conveyed to the mind. This intricate problem is solved in the most exquisite manner; moreover, in a manner that appears to be essentially the same in the case of the eye and the ear.

Let us approach this analogy with an illustration. When a piano is opened an attentive listener will readily discover that every time a note is sung in the room, that string of the piano which would have yielded the same note is thrown into sympathetic vibration. If the note be changed, another and corresponding string responds. Imagine now that a deaf man has his fingers lightly touching the strings of the piano; he will perceive when they vibrate and which vibrates. He will thus become conscious what note has been sung. A similar arrangement to the foregoing is fitted within each of our ears. In the inner ear there is a contrivance known as Corti's organ, which consists of no less than 3000 differently strained fibres. These fibres constitute the outward extremity of the auditory nerve: a vibration of any one of them immediately travels to the brain. According to one of the greatest living investigators, Helmholtz, it is believed that one or more of the fibres enters into sympathetic vibration whenever a sound reaches the ear. Corti's organ is to us, therefore, merely a refinement and extension of what the piano was to the deaf man. M. Hensen's experiments upon the means of hearing in the crustacea confirm this view.

As, however, it would be impossible for the strings of a piano to vibrate if the note were beyond the range of the instrument in either direction, so also there is a limit to the perception of sound by the ear. If the aerial waves recur more quickly than 38,000 times a second, no sound at all is heard, however intense the vibration. If the waves recur slower than sixteen times a second, they are inaudible as a continuous sound. Hence the extreme range of our hearing embraces about eleven octaves. This limit varies slightly in different persons, as first shown by Dr. Wollaston.* For example, some persons whose hearing is otherwise good cannot hear the chirp of a cricket or the squeak of a bat. Moreover, one ear is often more sensitive than the other,† and some ears are more sensitive to one class of sounds than to others.

Now, with regard to vision, a similar arrangement to that in the ear appears to exist at the back of the eye. The optic nerve there

* Phil. Trans.,' 1820, p. 306.

When travelling in Norway last year the writer observed that on one occasion the sounds from myriads of grasshoppers were heard, but on closing the left ear there was perfect silence: opening the left ear and closing the right ear, the sound was heard as loud as ever. The difference, where it exists, may readily be noticed by listening to the ticking of a watch. The cause, probably, arises from habitually sleeping on the right side.

spreads out to form the retina, upon the terminal filaments of which are scattered minute bodies, the so-called "rods and cones." Upon these bodies the luminous waves impinge, and it is considered probable that each accepts only that vibration which synchronizes with its own: in other words, that the perception of light arises from sympathetic vibration.* If this be the case, we should expect that the range of vision would, like hearing, fall within certain limits of pitch. This is well known to be the fact. Albeit the limit of vision is much more restricted than the limit of hearing; for with the utmost care we are unable to perceive vibrations of the ether beyond the range of an octave; that is, from the solar line A to L of the spectrum (see Plate). This extent needs, indeed, a practised eye; ordinarily the range corresponds to the interval termed a sixth in music, that is from the red to the violet extremity of the spectrum. The lower limit of vision, the extreme red, is produced by ethereal waves recurring 458 millions of millions of times each second; if the undulations be slower than this, they are invisible. The higher limit of vision, the extreme violet, is produced by luminiferous waves recurring 727 millions of millions of times each second; if faster, they do not excite the sense of sight. Inconceivable as is this rapidity, these figures are not hypothetical, nor merely probable; they express absolute facts incontestably established.

Like the limits of hearing, so these limits of vision vary slightly with different individuals; some people are capable of seeing farther beyond the red and not so far into the violet, whilst the converse is true with others. Hence, beyond the shadow of a doubt, certain sounds and certain lights perceived by some persons are totally unperceived by others. And when we pass from human beings to the larger animals on the one hand, and to insects on the other, we doubtless have the range both of hearing and of vision considerably extended. We are not aware that the limits of vision in animals have ever been studied; but analogy and experience lead us to suppose that it differs from our own in many cases. Assuming that the perception of light is due to a sympathetic vibration of the filaments of the retina, it merely needs that these filaments should be capable of vibrating only the one hundred millionth of a millionth per second slower, and what we call black heat would be perceived as light and if these filaments could vibrate the same amount faster, what we call the actinic or invisible chemical rays beyond the violet would become directly visible. Now it is highly improbable that the retina of every animal in creation should be, as it were, tuned to the same pitch as ours; and if this be so, then forces unrecognized by our senses are perceptible elsewhere.

The structure of the ear in a calf points to the conclusion *This suggestion was first made, we believe, by Melloni, in his 'La Thermochrose.'

that only the lowest sounds can be audible to that animal, but that its lower limit of hearing is beneath ours.* This is consistent with the doleful sounds made by that creature. Happily therefore for themselves, the lowing of cattle obviously produces a totally different impression upon their kindred to that which it produces upon us. Moreover, movements unheard by us are probably perceived as sound by them. If this be so at the lower limit of hearing, may not similar instances occur at the higher limit? It is perfectly conceivable, nay likely, that many insects produce and hear sounds far beyond our cognizance. And it would be most interesting to ascertain whether the organs of hearing in a bat, for example, or a grasshopper, correspond to the shrill sounds they produce.

Returning for one moment to the illustration of the sympathetic sounding of a piano, we find that when its strings are thrown into vibration the motion takes time to subside. Hence we should expect to find a lingering in our perception of both light and sound, after the exciting cause had ceased. This is exactly what occurs. The retention of light upon the retina amounts to th of a second. This retention causes a luminous point in rapid motion to appear as a line; the successive impressions blend into one. We cannot perceive as distinct from each other flashes of light that succeed each other at shorter intervals than the th of a second apart. Similarly our ear cannot distinguish between a succession of similar sounds that follow each other at shorter intervals than the th of a second. Like the blending into one of the colours on a spinning-top, the separate sounds link themselves together and constitute a musical note.

Radiant sound and light being both wave-motions, many laws are found common to both. When light falls upon a body it is either transmitted, absorbed, reflected, refracted, or inflected. The same phenomena can be observed when we substitute sound for light. Let us briefly examine this remarkable series of analogies.

§ 4. TRANSMISSION OF LIGHT AND SOUND.

Through transparent bodies light is transmitted freely. But if a series of transparent substances, each alternately differing in density, be placed together, the progress of light is obstructed, and may even be altogether stopped. Thus a perfectly transparent block of glass, if reduced to powder, is perfectly opaque. Air, a medium of different density, now intervenes between the particles of glass, and the light echoed, as it were, from particle to particle is so weakened that it cannot struggle through.

In the same way sound in its passage is enfeebled, or even obliterated, if it pass through several media of alternately varying

* Savart: 'Annales de Chimie et de Physique.'