3. Familiar Lectures on Scientific Subjects. By Sir John F. W. Herschel, Bart., K. H., M. A., etc. London: Alexander Strahan. 1867.

4. Faraday as a Discoverer. By John Tyndall. New York: D. Appleton & Co. 1868.

A fool can ask more questions than a wise man can answer. However far we may analyse any fundamental subject, we are compelled to pause at those outer limits which may be called the corner-posts of nature. The unsatisfactory results of many philosophical systems may doubtless be traced to the effort made to define those primitive elements of all definitions which can not, in the nature of things, be subject to limitations. Seneca well contrasts some of the dismal conclusions thus reached. 'If', says he, 'I believe Protagoras, there is nothing in nature but doubt; if Nausiphanes, this thing only is certain, that nothing is certain; if Parmenides, every thing is but one thing; if Zeno, every thing is nothing.' Nothing approaches, in august origin and abstruse natyre, more nearly to the elemental mystery of life itself, than light, the 'first-born of Heaven',-offspring, indeed, of the earliest recorded utterance of the creative Power. What is light?' is a question to which we may frankly reply, as to a thousand similar ones touching the primitive mysteries of the universe, that we do not know. Yet there is a great deal about light which we do know,- many most wonderful facts, out of which and for the explanation of which the mind strives to build up a reasonable theory of the nature of light. To define it as an agency subject to certain laws and producing such and such results, by no means satisfies the inquiring understanding. As yet, however, we can scarcely do more.

Three fundamental laws of light are as follows:

1°. Itself invisible, it renders all material objects within its sphere of action visible.

2o. For any given medium it acts in right lines, in all directions from the luminous body.

3°. This action proceeds at an enormous velocity.

The invisibility of light may strike one, at first, as a thesis out of Anaxagoras, who, according to Cicero, proved to the satisfaction of his own senses that snow is black. Nevertheless, it

is true. Take a box the inner walls of which are, like the chamber of a camera, thoroughly blackened. In one face puncture a pin-hole and admit a ray of light. If, through a blackened tube inserted in the upper side immediately over the line of the ray, we gaze down into the chamber, all is darkness. The light is there, however; for, on lowering by a thread through the tube a silvered bead into the line of the ray, its star-like reflection will instantly spring into view. 'A sunbeam, indeed,' says Sir John Herschel, 'is said to be seen when it traverses a dark room through a hole in the shutter, or when in a partially clouded sky luminous bands or rays are observed as if darted through openings in the clouds, diverging from the (unseen) place of the sun as the vanishing point of their parallel lines seen in perspective. But the thing seen in such cases is not the light, but the innumerable particles of floating dust or smoky vapor, which catch and reflect a small portion of it, as when in a thick fog the bull'seye of a lanthorn seems to throw out a broad, diverging luminous cone, consisting in reality of the whole illuminated portion of the fog.' (pp. 223-4.)

The rectilinear transmission of light is also proved by the phenomena here mentioned by Sir John, as well as by observations too familiar to need recital. We ascertain the rapidity of its transmission from more abstruse considerations. Ordinary terrestrial phenomena indicate that the communication of light is instantaueous, and for what we name 'practical purposes' this is so. As a fact, however, it requires time and is subject to a definite velocity.

Around the planet Jupiter, four satellites revolve in different orbits, nearly circular. The periodical times of their revolutions, as well as the dimensions and positions of both the satellites and their orbits, have been carefully and accurately determined. The three nearest to the planet move in orbits lying nearly in the plane of the path of the latter round the sun. Consequently, they suffer eclipse by the interposition of the body of the planet at every revolution. The observation of these eclipses being useful in the determination of longitudes of places on the earth's surface, the periods of their occurrence are now regularly calculated beforehand. But the times thus predicted,

upon data so thoroughly ascertained, were found to vary from the observed times,- being some times earlier, some times later, by a regular gradation of differences. In 1676, Roemer, a Danish astronomer, traced these discrepancies to their true cause. The eclipses took place too soon at the periods when the earth in its annual course came nearest to Jupiter, too late when it receded farthest. The total variation, amounting to sixteen minutes and twenty-six seconds, or not quite one thousand seconds, indicated, therefore, the time consumed by the light from the satellites in crossing the diameter of the earth's orbit. This diameter, heretofore taken at one hundred and ninety millions of miles, is now considered (from late observations upon the distance between the orbits of Mars and the earth) as more probably being about one hundred and eighty-four millions of miles in length, From these data the velocity of light appears to be about one hundred and eighty-six thousand, five hundred miles per second.

The discovery of the aberration of light by Dr. Bradley, in 1727, afforded a means of confirming this almost incredible result. Though we can not here enter upon a full explanation of this phenomenon, a conception of it may be had by considering the case of two men moving with rapidity in opposite directions during a shower of rain falling perpendicularly. The rain-drops will fall upon the faces of the two men as if proceeding in inclined lines from points in front of their respective zeniths. The rain-drops represent the rays of light in the astronomical phenomenon, and the opposing motions of the observer are those of the earth at the opposite sides of its orbit. The inclination of the rays is the result of the motion of light combined with the earth's orbital movement. The latter is known and the angle of inclination can be measured, and these data furnish, by an extremely simple calculation, an estimate of the velocity of light.

But the velocity of light has also been measured by means of mechanism, the principle of whose action may be said to be the subdivision of a second of time into very minute parts, in a word, the atomizing of time. M. Fizeau, of the French Academy of Sciences, effected this by means of a toothed wheel, in which the teeth were precisely of the same size as the intervals between them. The light of a lamp was directed through an

aperture in a screen so as to cross one of these intervals and fall upon a reflector placed at a known and considerable distance from the wheel. The reflector was so arranged as to throw back the beam through the notch in the wheel exactly opposite to that through which it first passed. Through this notch could be perceived the reflected ray, which had traversed a distance double that of the reflector from the wheel. When, now, the wheel was revolved with increasing velocity, the reflection at first seen continuously, gradually became feebler and presently entirely disappeared. This occurred when the velocity of the wheel was such that the light transmitted through the notch on one side was intercepted by the tooth adjacent to the opposite aperture on its return; that is, when the velocity of rotation carried a tooth over its own breadth whilst the ray was going and returning. This velocity is readily measured; indeed, it may be registered by the mechanism used to drive the wheel. If the wheel, as was actually the case, makes twelve and six-tenths revolutions in a second and has fourteen hundred and forty divisions (teeth and notches), the time of the passage of a tooth across its own breadth is found by taking the reciprocal of the product of these numbers. In this fraction of a second the ray has traversed twice the distance between the mirror and the wheel, which amounted in M. Fizeau's experiments to eighteen thousand, eighteen hundred and eighty yards. But this total distance measured or divided by the time, will give the distance gone over in a second, or, in other words, the velocity of the ray. The mean results of the experiments established a velocity of one hundred and ninety six thousand miles.

The far more refined and delicate method of M. Foucault has shown, however, that this result is too great. This method is essentially that first used by Arago, in an experiment determining the relative velocities of light in air and water. A horizontal ray of light is admitted into a darkened chamber, and falls upon a mirror arranged to revolve on a vertical axis lying in its own plane. As the mirror turns, the reflected ray will move, of course, in a horizontal plane passing through the point of incidence and the aperture of admission, and by an easy geometrical consideration its angular velocity is known to be double that

of the mirror. In this horizontal plane, a second mirror is placed perpendicular to a line itself drawn perpendicular from the centre of the last to the axis of the first;-placed, in other words, so as to return a ray to the first mirror upon the same path in which it is first reflected from it. If, now, the revolving mirror be supposed at rest and be so arranged as to reflect the ray (received through the aperture) upon the second mirror, the ray will manifestly be returned by the latter upon the same path, and will be again reflected by the first directly towards the aperture. But if, whilst the ray has been passing between the mirrors, the first has revolved through a small angle, the ray in passing back towards the aperture will deviate from its original path by an angle double that described by the mirror. This angle is readily measured; and the fraction of a second required by the light to traverse the distance between the mirrors, to and fro, multiplied by the angle described by the mirror in any small fraction of a second taken as a unit, will give a product equal to one-half this measured angle. But the number of rotations in a second being registered, the unit angle may be readily deduced from the unit of time. The product and one factor of it being thus known, we derive the other factor, or the time of the ray; this, with the space passed over the double distance between the mirrors-affords one factor of another known product; so that finally dividing the space by the time we obtain the velocity.

We may here mention, in passing, that by the interposition of a column of water between the mirrors, through which the ray is passed, we have the means of ascertaining the velocity of the propagation of light through water, and that this is found to be less than its velocity in air.

The result of M. Foucault's experiments was a velocity of 185,172 miles; so that taking all the methods into consideration, neglecting only M. Fizeau's as subject to important errors from mechanical imperfections, we may conclude that the velocity of light in interplanetary and cosmical space is about one hundred and eighty-six thousand miles in a second!

This enormous velocity takes hold upon the infinite, and is beyond any adequate comprehension. The greatest speed we