Abbildungen der Seite
PDF
EPUB

I am enabled by the convexity of an eye-glass to view that image so closely, as that the visual angle is increased to 40 degrees, the effect is the same as if I had advanced within 10 yards of the tree, when its angle would, to the naked eye, have been increased to 40 degrees. The tree is consequently magnified by the eye-glass 40 times in height and breadth.

2.-In a microscope, as the object itself can be brought so near, as to serve the purpose of the image in the telescope, the single microscope is but one lens, but the compound microscope is an arrangement of eye-glasses ; which, in some of them, enables the eye to view the image within the 50th part of an inch; thereby enlarging the object in comparison with the natural vision, at six inches distance, 300 times in length and breadth, or 9000 times in surface.

THE TELESCOPE.

[ocr errors]
[ocr errors]

A
C

M Y
A D is the Object-Glass, or glass nearest the object.
E Y the eye-glass.
K is the eye of the observer.
O B is the object or scene to be viewed.

OC B is the angle under which the remote object is seen by the naked eye.

ICM is the angle of the optical image produced by the object-glass, and is equal to the angle O C B, being produced by the crossing of the same lines.

The glass E Y enables the eye to see the image I M under the angle E K Y, which is the same as E P Y.

Those angles are, however, in the inverse ratio of the distances C P and PG; or, in other words, as the focal distances of the object and eye-glasses, and such, therefore, is the power of the telescope generally.

In such a glass, the image will be reversed to the eye, but by adding two other eye-glasses, it is set straight again. In Gallileos, however, only one eye-glass is need.

ful. In viewing the heavenly bodies, the reversal is of no consequence.

572. As the image is reversed by a single lens, owing to the crossing of the rays in the centre of. the lens, the magnified image is reversed when viewed with one eye-glass; two other glasses are therefore added; one for the purpose of restoring the image to its natural position ; and the other for the purpose of viewing it as at first.

The lens within the eye, reverses objects; but the mind contemplates the top of the optic nerve, as corresponding with the bottom of the object; and the mental effect, is the result of habit, or of learning to see in infancy, or after the eyes have been couched in manhood.

573. Telescopes are refractors, when the effect is produced solely by refraction through transparent lenses; or reflectors, when the image is produced by the converging rays of a concave mirror ; but the principle of the magnifying power is the same in all.

They are called Gallileos when the eye-glass is concave, after Gallileo, the inventor of telescopes, who made most of his discoveries in astronomy by a small telescope, which magnified but 12 times; whereas Herschel's great telescope mag. nifies 6000 times !

574. Of course, without light, the world would be involved in total darkness; and without the mechanism of eyes, and optic nerves to convey the varied sensations of light to the brain, all the beauties of nature derived from its diversity of colouring, all the interesting relations of day and night, and half the pleasures of existence, would be totally lost.

The cause of the various colours which adorn the creation is, of course, an object of interesting inquiry; and this discovery was the greatest of those made by Newton.

The beams of light had been, in vain, display'd,
Had not the eye been fit, for vision made;
In vain, the Author had the eye prepar’d,
With so much skill, had not the light appear'd

BLACKMORE. 575. Every one observes the beautiful colours produced by the pendant drops of cut-glass hanging to lustres and chandeliers, whether derived from candle-light or sunshine.

It is observed too, that drops of rain are coloured in like manner, when the sun shines upon them;

and the regular form of the colours in the rainbow, led Newton to conclude, that these colours, as well as colours in general, were produced by some property in rays of light.

576. Newton made a beam of sunshine pass through a hole in a window-shutter, and fall on a glass prism or wedge, so as to be refracted out of its course towards the ground, and thrown upwards on the opposite wall.

He then found, that the circular beam of light was rendered oblong and regularly coloured; that the uppermost part, or the rays most refracted, were violet, their next division indigo, the next blue, then green, yellow, orange, and at bottom red, being the seven colours of the rainbow.

Of parent colours, first, the flaming red
Sprung vivid forth ; the tawny orange, next;
And next, delicious yellow ; by whose side,
Fell the kind beams of all-refreshing green ;
Then the pure blue, that swells autumnal skies,

Ethereal play'd ; and then of sadder hue,
Emerged the deepen'd indigo, as when
The heavy skirted evening droops with frost;
While the last gleamings of refracted light
Dy'd, in the fainting violet, away.-BLACKMORE.

THE PRISM AND ITS COLOURS.

DB

Violet
Indigo
Blue
Green
Yellow
Orange
Red

E represents the shutter of a room.
D a hole in the shutter.

S rays of light proceeding from the sun passing through the hole, and falling on the glass prism B AC; on meeting which at B C, instead of going straight on, it is refracted, and leaves the prism at A C.

T is its figure on the opposite wall, spread from a circle to an oblong, and presenting the colours of the re. fracted rays in succession as marked.

577. Å beam of white light is, therefore, found to consist of rays of all the colours; and it is evident, that the various colours of all the bodies in nature, depend solely on the power of the surfaces to absorb some rays, and reflect others.

It appears, too, that white is a due mixture of the seven primary colours wholly reflected ; that black objects absorb all the rays, reflecting none; and that black is an effect of negation.

Colours are but the phantoms of the day ;-
With that they're born, with that they fade away;
Like beauty's charms, they but amuse the sight,
Dark in themselves, till, by reflection bright,
With the sun's aid, to rival him they boast;
But light withdrawn, in their own shades they're lost.

HUGHES. 578. Hence, white bodies in the sun are cool, and black ones hot; because white surfaces reflect all the light, and black ones absorb it: hence also, as red rays are the least refracted, they are supposed to be the largest, and are therefore, painful to the eye; and hence, the most refrangible rays, as the smallest, are the most grateful.

Obs.-When similar thermometers are placed in the different parts of the solar beam, separated by the prism, it is found, that different effects are produced in the dif ferent coloured rays. The greatest heat is exhibited in the red rays: the least in the violet rays; and in a space beyond the red rays, where there was no visible light, the increase of temperature is greatest of all. This important discovery was made by Dr. Herschel. He estimates the power of heating in the red rays, to be to that of the green rays as 55 to 26; and to that of the violet rays as 55 to 16. A thermometer, in the full red

rays,

indicated an increase of temperature of 7° Fahrenheit, in ten minutes beyond the red rays, in an equal time, the increase was 9° Fahrenheit. From these facts, it is evident, that matter set in motion by the sun, has the power of producing heat without light, and that its rays are less refrangible than the visible rays. Rays capable of producing heat with and without light, proceed from bodies at the surface of the globe under peculiar agencies or changes, as well as from the sun ; and the phenomena,

hat are usually called the phenomena of the radiation of terrestrial heat, are of great extent and importance, and well worthy of being studied. There is another fact, still more extraordinary, which has been called the radiation et

Y

« ZurückWeiter »