« ПретходнаНастави »
ihe 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 the 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 incbes distance, 300 times in length and breadth, or 9,000 times in surface.
OC B is the angle under whïch the remote object is seen by the naked eye.
ICM is the angle of the optical image produced by the object-glase, 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 KY, which is the same as EP Y.
Those angles are, however, in the inverse ratio of the distances E 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 needful. In viewing the heavenly bodies, the reversal is of no consequence.
588. 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 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.
589. 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 all his discoveries in astronomy by a small telescope, which magnified but 12 times; whereas Herschel's great telescope magnifies 6,000 times !
590. 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; 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 visión made : In vain, the Author had the eye prepar'd, With so much skill, had not the light appear’d. 591. Every one observes the beautiful colours produced by the pendent drops of cut glass hanging to lustres and chandeliers, whether derived from candlelight 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 rays
of light. 592. 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
$ 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 refracted says in succession as marked
593. A 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;
594. 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 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 dif. ferent parts of the solar beam, separated by the prism, it is found, that different effects are produced in the different 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 is no visible light, the increased 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° Fah. renheit, in ten minutes ; beyond the red rays, in an equal time, the increase was 90 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 re
frangible 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 that are usually called the phenomena of the radiation of lerrestrial 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 of cold, first observed by the Italian philosophers, and afterwards by Pictet. If in the arrangement of the two parallel mirrors, a piece of ice be introduced into the lower focus, the thermometer in the upper focus will indicate a diminution of temperature.
595. Rainbows arise from the rays of the sun which falls on drops of water being reflected and refracted to the eye of the spectator ; and, of course, all those drops which are situated at the same angle all round from the eye, will present the same colour.
And, as different colours will arise at different angles; a bow composed of regular circles is a necessary consequence of showers of rain, while the sun shines.
It will, however, only be visible opposite to the sun; and the line from the sun through the eye of the spectator must be its centre.
596. As rays will reach the eye from drops of rain: owing to two different causes; so there will generally be two rainbows, one fainter, however, than the other.
The strongest, or lowermost rainbow, is occasioned by the light being reflected from the upper part of the back of the drops of rain; and the other, or upper bow, is occasioned by the light being twice reflected within the drop from the lower part to the upper,
and thence refracted to the eye.
597. The breadth of a rainbow is about two de grees; and, of course, no two spectators can see the same rays; but every eye will be the centre of its own bow.
All circles round the sun and moon arise in like manner, from the peculiar modifications of the rays