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Of the same kind a meteor, called a falling star, which is a luminous point in rapid motion — the motion of a rocket, etc.
The following is a very instructive experiment: Take a circular disc of white pasteboard, or perhaps better, paste white paper on a circular piece of board, and having divided the surface into sections of proper proportions, and painted on them the prismatic colours when made to revolve rapidly it will appear white - if whirled round in a dark room, and with the same rapidity which before produced white, when lighted by an electric spark, all the colours are as distinctly visible as if the wheel were at rest; in this case the wheel has moved through no visible angle, while the light lasted, and may be taken to have been at rest; if lighted by a flash from gunpowder, they will be less distinct, but here the duration of light is longer.
“ It has generaliy been supposed, since the time of Newton, that when the rays of light are separated as completely as possible by means of refraction, they exhibit seven varieties of colour, relating to each other with respect to the extent that they occupy in ratios nearly analogous to those of the ascending scale of the minor mode in music. The observations were, however, imperfect, and the analogy wholly imaginary. Dr. Wollaston has determined the division of the coloured image or spectrum in a much more accurate manner than had been done before; by looking through a prism, at a narrow line of light, he produces a more effectual separation of the colours than can be obtained by the common method of throwing the sun's image on a wall. The spectrum proved in this manner consists of four colours only, red, green, blue, and violet, which occupy spaces in the proportion of 16, 23, 36, and 25, respectively, making together 100 for the whole length; the red being nearly one sixth, the green and violet each about one fourth, and the blue more than one third of the length,”Young's Lectures.
Transparent substances, as glass, may be made into such forms that the light falling on them, after passing through, may be brought to a point at particular distances.
The eye is of this nature, and it collects the light which falls upon it from objects around, and brings them to a point on what is called the retina— when they are exactly brought to a point there the sight is good ;-— when the surface of the eye is too round, the image is not in its proper place, and as people get older, in the generality of cases, the eye becomes too flat; – to assist them in both cases, lenses (when used in this way called spectacles) are had recourse to, and by the assistance of these, the image is formed at the proper point; – when the eye is too flat, the image is behind the retina, when too round, between the retina and the eye; but in neither case can people see well.
Short-sighted people have the eye too convex, longsighted too flat; this latter defect comes with age, or increases as people get older, which is the reason why they cannot read without spectacles.
This does not increase the quantity of light, as light is lost in passing through the spectacles.
The effort which every one whose sight is beginning to fail feels himself making in order to read, or see anything which is indistinct, is to bring the lens of the eye into such a form, that the image may be formed in its proper place.
Then a teacher would ask them if they had never . observed the effect of going out from a lighted room on a dark night, how little they could see at first, and the sort of muscular action going on in the eye, so as to adjust it to collect more light; - the contrary, going from dark to great light, as in opening the shutters of a bedroom window on a bright morning, causing a sort of involuntary effort of the eye to contract, and exclude part of the light — reflection of light from snow causing pain, etc.
Owls, etc., and animals which see well at night having the power of dilating the pupil of the eye, so as to take in . more light.
To have a perfect sight, the lens of the eye must be so shaped that the image is formed exactly on the retina.
Sir John Herschel, in his “Discourse on the Study of Natural Philosophy,” mentions, among others not less striking, the following instance of theory and pure mathematical analysis leading to results such as no ordinary practical reasoning would be able to get at, being contrary, as it were, to one's every-day experience.
“An eminent living geometer had proved by calculations founded on strict optical principles, that in the centre of the shadow of a small circular plate of metal, exposed in a dark room to a beam of light emanating from a very small brilliant point, there ought to be no darkness - in fact, no shadow, at that place; but on the contrary, a degree of illumination precisely as bright as if the metal plate were away. Strange and even impossible as this conclusion may seem, it has been put to the trial and found perfectly correct.
“ Cases like this,” he justly adds, " are the triumph of theories.” -- HERSCHEL'S Discourse on Nat. Phil.
There are a few facts connected with Astronomy, and, when properly explained, not very difficult to comprehend, which ought to form a part of the instruction given in our schools.
The apparent motion of the heavenly bodies -- that this is caused in part by a real motion of the spectator, which he himself is not aware of — that the movements we see of the sun, and among the stars, are not all real ones, but owing to our point of view changing every moment.
That all these bodies appearing to be in a blue concave sphere on a fine night, and at nearly equal distances from us, are not really so that some are millions and millions of miles farther from us than others — some are fixed and do not change their position with respect to each other, and are called fixed stars — others, again, are mor. ing in circular orbits round the sun, in the same manner as the earth does, of which a certain number are known — their distances from the sun — the time of revolving in their orbits accurately calculated; that is, the time from one of these bodies leaving any one point in its orbit until it comes to the same point again – these are called planets - some of them, again having satellites or moons revolving round them, in the same way as the moon round our earth.
Again, that some of them are self-luminous bodies, like the sun, as the fixed stars - others like our moon, are not in themselves luminous, but appear to be so by reflecting the light thrown upon them by the sun — this explains the various phases of the moon, new moon, full moon; -otherwise, if she were a luminous body, she would always appear the same, etc.
These and similar things which they may be taught are no doubt quite opposed to their preconceived notions, so far as they may have notions at all, or have ever thought on the subject; but I can say, from my own experience, that when explained in a simple way they excite a very lively interest, and are not only highly instructive as to the facts themselves, but may be made a means of imparting to the youthful mind strong feelings of a religious character.
I saw the glorious sun arise
In morning's early gray,
And melt the shades away.
The heavens to adorn?
And gave us back the morn?
The heavens to adorn;
And gave us back the morn.
Having become acquainted with the different lines on the surface of a terrestrial globe, * they should be made to understand the two motions of the earth, one in its orbit causing the variations of the seasons, the other of rotation, causing day and night, and that this motion on its axis from
* “This earth of ours is a huge mass, self-poised, supported upon nothing, hung upon nothing --enveloped by the air which we breathe, and surrounded by the space of the heavens. “How many thoughts does the mind embrace in this idea!”
west to east causes an apparent motion of the sun and stars from east to west.
Turning the globe from west to east (having first elevated the pole to the latitude of the place, it is easily understood that a point on the surface near the pole describes a very small circle, and that every point which is more distant describes a larger one, till we reach the equator, any point on which describes a great circle, and that from the equator to the south pole these circles go on decreasing.
Hence the teacher would call attention to the tendency which a body would have to fly off from the surface of the earth, caused by this rotation — that the more rapid the motion, the greater this tendency—that the motion being greatest at the equator and decreasing towards the poles, this tendency to fly off, would be greater there than at any other point; and would in all cases diminish the weight of bodies, and that this was found by experience to be the case ; a body at the equator loses from this any of its weight.
This tendency to fly off is always at right angles to a perpendicular to the axis of rotation, and at the equator is at right angles to the direction of gravity.
The centrifugal force at any point on the earth's surface acts at right angles to a perpendicular let fall from that point on the axis of rotation, and varies in magnitude as that perpendicular which is the cosine of the latitude; at the equator this force is at right angles to the direction of gravity, and is a maximum, the latitude being 0, and the cosine equal to radius; at the pole it is nothing, the latitude being 90°, and cosine of 90° is 0.
Why is a bird in its flight not left behind by this rotation of the earth on its axis ? or, why does not the lark soaring in the sky find the field moved from under her when she descends?
He might then instance the dirt or wet flying from a cart or carriage-wheels in rapid motion over dirty roads — the water from a wet mop when twirled round from a grindstone when the blacksmith is grinding tools ;—then to show how easy it is, from knowing the properties of a circle, to calculate the absolute space moved through by