The Practical Astronomer. Thomas Dick

of modern astronomers have demonstrated that this is an erroneous hypothesis, and that light, like other projectiles, occupies a certain time in passing from one part of space to another. Its velocity, however, is prodigious, and exceeds that of any other body with which we are acquainted. It flies across the earth’s orbit—a space 190 millions of miles in extent, in the course of sixteen and a half minutes, which is at the rate of 192,000 miles every second, and more than a million of times swifter than a cannon ball flying with its greatest velocity. It appears from the discoveries of Dr. Bradley, respecting the aberration of the stars, that light flies from those bodies, with a velocity similar, if not exactly the same; so that the light of the sun, the planets, the stars, and every luminous body in the universe is propagated with uniform velocity.3 But, if the velocity of light be so very great, it may be asked, how does it not strike against all objects with a force equal to its velocity? If the finest sand were thrown against our bodies with the hundredth part of this velocity, each grain would pierce us as certainly as the sharpest and swiftest arrows from a bow. It is a principle in mechanics that the force with which all bodies strike, is in proportion to the size of these bodies, or the quantity of matter they contain, multiplied by the velocity with which they move. Therefore if the particles of light were not almost infinitely small, they would, of necessity prove destructive in the highest degree. If a particle of light were equal in size to the twelve hundred thousandth part of a small grain of sand,—supposing light to be material—we should be no more able to withstand its force than we should that of sand shot point blank from the mouth of a cannon. Every object would be battered and perforated by such celestial artillery, till our world were laid in ruins, and every living being destroyed. And herein are the wisdom and benevolence of the Creator displayed in making the particles of light so extremely small as to render them in some degree proportionate to the greatness of the force with which they are impelled; otherwise, all nature would have been thrown into ruin and confusion, and the great globes of the universe shattered to atoms.

      We have many proofs, besides the above, that the particles of light are next to infinitely small. We find that they penetrate with facility the hardest substances, such as crystal, glass, various kinds of precious stones, and even the diamond itself, though among the hardest of stones; for such bodies could not be transparent, unless light found an easy passage through their pores. When a candle is lighted in an elevated situation, in the space of a second or two, it will fill a cubical space (if there be no interruption) of two miles around it, in every direction, with luminous particles, before the least sensible part of its substance is lost by the candle:—that is, it will in a short instant, fill a sphere four miles in diameter, twelve and a half miles in circumference, and containing thirty-three and a half cubical miles with particles of light; for an eye placed in any part of this cubical space would perceive the light emitted by the candle. It has been calculated that the number of particles of light contained in such a space cannot be less than four hundred septillions—a number which is six billions of times greater than the number of grains of sand which could be contained in the whole earth considered as a solid globe, and supposing each cubic inch of it to contain ten hundred thousand grains. Such is the inconceivable tenuity of that substance which emanates from all luminous bodies, and which gives beauty and splendour to the universe! This may also be evinced by the following experiment. Make a small pin-hole in a piece of black paper, and hold the paper upright facing a row of candles placed near each other, and at a little distance behind the black paper, place a piece of white pasteboard. On this pasteboard the rays which flow from all the candles through the small hole in the black paper, will form as many specks of light as there are candles, each speck being as clear and distinct as if there were only one speck from a single candle. This experiment shows that the streams of light from the different candles pass through the small hole without confusion, and consequently, that the particles of light are exceedingly small. For the same reason we can easily see through a small hole not more than ¹/₁₀₀th of an inch in diameter, the sky, the trees, houses, and nearly all the objects in an extensive landscape, occupying nearly an entire hemisphere, the light of all which may pass through this small aperture.

      3. Light is sent forth in all directions from every visible point of luminous bodies. If we hold a sheet of paper before a candle, or the sun, or any other source of light, we shall find that the paper is illuminated in whatever position we hold it, provided the light is not obstructed by its edge or by any other body. Hence, wherever a spectator is placed with regard to a luminous body, every point of that part of its surface which is toward him will be visible, when no intervening object intercepts the passage of the light. Hence, likewise, it follows, that the sun illuminates, not only an immense plane extending along the paths of the planets, from the one side of the orbit of Uranus to the other, but the whole of that sphere, or solid space, of which the distance of Uranus is the radius. The diameter of this sphere is three thousand six hundred millions of miles, and it, consequently, contains about 24,000,000,000,000,000,000,000,000,000, or twenty-four thousand quartillions of cubical miles,—every point of which immense space is filled with the solar beams. Not only so, but the whole cubical space which intervenes between the sun and the nearest fixed stars is more or less illuminated by his rays. For, at the distance of Sirius, or any other of the nearest stars, the sun would be visible, though only as a small twinkling orb; and consequently, his rays must be diffused, however faint, throughout the most distant spaces whence he is visible. The diameter of this immense sphere of light cannot be less than forty billions of miles, and its solid contents 33,500,000,000,000,000,000,000,000,000,000,000,000,000 or, thirty-three thousand, five hundred sextillions of cubical miles. All this immense, and incomprehensible space is filled with the radiations of the solar orb; for were an eye placed in any one point of it, where no extraneous body interposed, the sun would be visible either as a large luminous orb, or as a small twinkling star. But he can be visible only by the rays he emits, and which enter the organs of vision. How inconceivably immense, then, must be the quantity of rays which are thrown off in all directions from that luminary which is the source of our day! Every star must likewise be considered as emitting innumerable streams of radiance over a space equally extensive, so that no point in the universe can be conceived where absolute darkness prevails, unless in the interior regions of planetary bodies.

      4. The effect of light upon the eye is not instantaneous, but continues for a short space of time. This may be proved and illustrated by the following examples:—If a stick—or a ball connected with a string—be whirled round in a circle, and a certain degree of velocity given it, the object will appear to fill the whole circle it describes. If a lighted firebrand be whirled round in the same rapid manner, a complete circle of light will be exhibited. This experiment obviously shows that the impression made on the eye by the light from the ball or the firebrand—when in any given point of the circle—is sufficiently lasting to remain till it has described the whole circle, and again renews its effect, as often as the circular motion is continued. The same is proved by the following considerations:—We are continually shutting our eyes, or winking; and, during the time our eyes are shut, on such occasions, we should lose the view of surrounding objects, if the impression of light did not continue a certain time while the eye-lid covers the pupil; but experience proves that during such vibrations of the eye-lids, the light from surrounding objects is not sensibly intercepted. If we look for some time steadily at the light of a candle, and particularly, if we look directly at the sun, without any interposing medium, or if we look for any considerable time at this luminary, through a telescope with a coloured glass interposed—in all these cases, if we shut our eyes immediately after viewing such objects, we shall still perceive a faint image of the object, by the impression which its light has made upon our eyes.

      ‘With respect to the duration of the impression of light, it has been observed that the teeth of a cog-wheel in a clock were still visible in succession, when the velocity of rotation brought 246 teeth through a given fixed point in a second. In this case it is clear that if the impression made on the eye by the light reflected from any tooth, had lasted without sensible diminution for the 246th part of a second, the teeth would have formed one unbroken line, because a new tooth would have continually arrived in the place of the interior one before its image could have disappeared. If a live coal be whirled round, it is observed that the luminous circle is complete, when the rotation is performed in the (8½)/60th of a second. In this instance we see that the impression