« ForrigeFortsæt »
SOME OBSERVATIONS ON THE NATURE OF
LIGHT AND COLOURS,
product of wisdom, and to rear up an im- ought to contain at its emergence the same potent agency into a work which Almighty proportional quantity of all the coloured Puwer could alone perfecto it is, in fact, to rays, because the part reflected at its antesink Omnipotence and Omniscience into rior and posterior surfaces ris colourless ; the mere revolutions of ages, or senseless but, in point of fact, such perfect want of progressions and operations of dead matter; colour in the transmitted beam is never which, instead of order, beauty, and sta- observed. Media, therefore, are unequally bility, fail not, on all occasions, to pro- transparent for the differently coloured rays. duce incongruity, disorder, weakness, and Each ray of the spectrum has, for every ruin. Infinite power and infinite wisdom different medium in nature, its own pecuare equally observable throughout creation; liar index of transparency, just as the index and no where are they more observable of refraction differs for different rays and than in the crust of the sphere assigned by different medias the Great Creator to us, for our habitation The following simple experiment shews, and solace. Here then, while we survey in a striking manner, the different absorpthe work, let us look up to and adore the tive power of one and the same medium Creator, whose presence and activity were on differently-coloured rays. Look through not more needful during the creation, than a plain piece of blue glass, (such as sugarthey are during the ages of His divine pro- basins, and finger-glasses are often made vidence over all that He hath called into of,) at the image of any narrow line of existence : “ For of Him, and through light, (as the crack in a window-shutter of Him, and to Him, are all things : to whom a darkened room,) refracted through a be glory for ever. Amen.
prism, whose edge is parallel to the line, King Square, Nov 20, 1831.
and placed in its situation of minimum W. COLDWELL. deviation. If the glass be extremely thin,
all the colours are seen; but if of moderate thickness, (as one twentieth : part of an inch,) the spectrum will put on a very
singular and striking appearance. It will (Concluded from p. 90.)
appear composed of several detached porTHOUGH no body in nature be perfectly, tions, separated by broad and perfectly all are to a certain degree, transparent. black intervals, the rays which correspond One of the densest of metals, gold, may to those points in the perfect spectrum actually be beaten so thin as to allow light being entirely extinguished. If a Jess to pass through it; and, that it passes' thickness be employed, the intervals, inthrough the substance of the metal, not stead of being entirely dark, are feebly and through cracks or holes too small to be irregularly illuminated, some parts of them detected by the eye, is evident from the being less enfeebled than others. If the colour of the transmitted light, which is thickness, on the other hand, be increased, green, even when the incident light is the black spaces become broader, till at white. All coloured bodies, however length all the colours intermediate between deep their hues, and, however seemingly the extreme red and extreme violet, are opaque, must necessarily be rendered totally destroyed. visible by rays which have entered their The simplest hypothesis we can form of the substance ; for, if reflected at their surfaces, extinction of a beam of homogeneous light, in they would all appear white alike. Were passing through a homogeneous medium, the colours of bodies strictly superficial, no is, that for every thickness of the medium variation in thickness could affect their passed through, an equal aliquot part huę; but, so far is this from being the case, of the rays, which up to that depth had that all coloured bodies, however intense escaped absorption, is extinguished. Thus, their tint, become paler by diminution of if one thousand red rays fall on' and enter their thickness.
into a certain green glass, and if one bunThis gradual diminution in the intensity dred be extinguished in traversing the first of a transmitted ray, in its progress through tenth of an inch, there will remain nine imperfectly transparent media, is termed hundred which have penetrated so far; its absorption. It is never found to affect and of these one-tenth, or ninety, will be equally rays of all colours, some being extinguished in the next tenth of an inch, always absorbed in preference to others; leaving eight hundred and ten, out of which, and it is on this preference that the colours again, a tenth, or eighty-one, will be exof all such media, as seen by transmitted tinguished in traversing the third-tenth, light, depend. A white ray transmitted leaving seven hundred and : twenty-nine, through a perfectly transparent medium, and so on.
When a ray of white or solar light falls refraction, may be received on a third obliquely on the surface of a refracting me
But it is now observed to be no dium, it is not refracted entirely in one longer separated into a coloured spectrum, direction, but undergoes a separation into like the original one of which it formed a several rays, and is dispersed over an angle part. A single spot only is seen on the more or less considerable, according to the screen, the colour of which is uniform, and nature of the medium, and the obliquity of precisely the same as that portion of the incidence. The several rays of which the spectrum from which it is taken. It apdispersed beam consists, are found to differ pears, then, that the ray which goes to form essentially from each other, and from the any single point of the spectrum, is not only incident beam, in a most important phy- independent of all the rest, but, having been sical character. They are of different once insulated from them, is no longer colours. The light of the sun is white, capable of further separation into different and if a sun-beam be admitted into a dark- colours, by a second refraction. ened room through a small round hole in From the above-nientioned simple expethe window-shutter, and be received directly ments, the following properties of light may on a piece of paper, it makes on it a round be deduced. white spot, which will be larger as the 1. A beam of white or solar light conpaper is further removed. To shew the sists of a great and almost infinite variety of separation or dispersion of the rays, take a rays, differing from each her in colour and triangular prism of good fint glass, and refrangibility. place it in the beam with one of its angles 2. White light may be decomposed, downwards, so that the beam may fall on analyzed, or separated into its elementary one of its sides obliquely. The beam will coloured rays by refraction. The act of then be refracted, and turned out of its such separation is called the dispersion of course, and thrown upwards, and may be the coloured rays. received on a screen properly placed. But 3. Each elementary ray, once separated on this screen there will no longer be seen and insulated from the rest, is incapable of a round white spot, but a long streak, or, further decomposition or analysis by the as it is called in optics, a spectrum of most same means. For we may place a third, vivid colours. The tint of the lower or and a fourth prism in the way of the twice least refracted extremity will be a brilliant refracted ray, and refract in any way, or in red, then an orange, afterwards a pale straw any plane ; it remains undispersed, and yellow, succeeded by a pure and very preserves its colour quite unaltered. intense green, which passes to a blue, at 4. The dispersion of the coloured rays first greenish, but, as the distance increases, takes place in the plane of the refraction ; deepening into the purest indigo, and, as for it is found that the spectrum is always the intensity of the illumination diminishes elongated in this plane. fading into a pale violet.
That the term analysis or decomposition, If the screen on which the spectrum be is correct as applied to the separation of a received have a small hole in it, only large beam of white light into coloured rays, may enough to allow a very narrow portion of be proved by the following experiment, in the spectrum to pass, and this portion of which, by a synthesis or joining together of the beam be received on another screen, the elementary rays, white light is again placed at some distance behind it, it will produced. there form a spot of the very same colour If a small circular beam of solar light be as that portion of the spectrum allowed to passed through a prism, and the dispersed pass. Thus, if the hole be placed in the coloured rays received in a lens at some red part of the spectrum, the spot will be distance, and transmitted to a white screen, red, if in the green, green; and in the the whole spectrum, instead of being coblue, blue. If the eye be placed so as to loured, will be re-united in a spot of white see through this small hole, an image of light. the sun will be beheld, of dazzling bright- That the re-union of all the coloured ness, not, as usually, white, but of the colour rays is necessary to produce whiteness, may of that portion of the spectrum which goes be shewn by intercepting a portion the to form the spot on the screen.
spectrum before it falls on the lens. Thus, If, instead of receiving the ray transmitted if the violet ray be intercepted, the white through the small hole in the first screen on spot will acquire a tinge of yellow; if the a second screen immediately behind it, it blue and green be successively stopped, be intercepted by another prism, it will be the yellow tinge will grow more and more refracted and bent from its course, as in ruddy, and pass through orange to scarlet the first instance; and, after this second and blood red. If, on the other hand, the 2D, SERIES, NO. 15.-VOL. II.
139.- VOL. XIV,
red end of the spectrum be stopped, and employ hollow prisms filled with water, or more and more of the less refrangible por- some of the more dispersive oils. If these tion thus successively abstracted from the are not at hand, the inconvenience may be beam, the white will pass first into pale, diminished by transmitting the ray as near and then to vivid green, blue green, blue, the edge of the prism as possible, so as to and finally into violet. If the middle por- lessen the quantity of the material it has to tion of the spectrum be intercepted, the re- pass through, and therefore the chance of inaining rays, concentrated, produce various encountering striæ in its passage. shades of purple, crimson, or plum-colour, It has been observed, that there are many according to the portion by which it is thus crystallized minerals, especially the tourrendered deficient from white light; and maline, which when cut into parallel plates by varying the intercepted rays, any variety are sufficiently transparent, and let pass of colours may be produced ; nor is there abundance of light with perfect regularity, any shade of colour in nature which may nevertheless, the light at its emergence is not thus be exactly imitated, with a bril- found to have acquired a peculiar modifiliancy and richness surpassing that of any cation, which has been termed polarity, or artificial colouring.
polarization. The difference between a According to the Newtonian doctrine of polarized and an ordinary ray of light can the origin of colours, and every phenome- hardly be more readily explained than by non in optics conspires to prove the truth assimilating the latter to a cylindrical, and of it
the former to a four-sided prismatic rod, “ The colours of natural bodies are not or flat ruler; that is to say, the polarized qualities inherent in the bodies themselves, rays seems to have acquired sices, and to by which they immediately affect our be rendered incapable of passing through sense, but are mere consequences of that certain media permeable to it in its original peculiar disposition of the particles of each or unpolarized state, as a broad flat ruler body, by which it is enabled more copi. will not pass through the bars of a narrow ously to reflect the ray of one particular grating, if presented to it crossways. colour, and to transmit, or stifle, or, as it is The following experiment will exemplify called in optics, absorb the others.” the thing clearly.
Perhaps the most direct and satis. The tourmaline, which is a species of factory proof of the truth of this doc- schorl, crystallizes in long prisms, whose trine is to be found in the simple fact, primitive form is the obtuse rhomboid, that every body indifferently, whatever be having its axis parallel to the axis of its colour in white light, when exposed
The lateral faces of these in the prismatic spectrum, appears of prisms are frequently so numerous as to the colour appropriate to that part of the give them an approach to a cylindrical or spectrum in which it is placed ; but that cylindroidal form. Now, if one of these its tint is incomparably more vivid and crystals be taken and slit (by the aid of a full, when laid in a ray of a tint analo- lapidary's wheel) into plates parallel to gous to its hue in a white light, than in the axis of the prism of a moderate and any other. . For example, vermilion placed uniform thickness, (about one twentieth in the red rays appears of the most vivid of an inch,) which must be well polished, red ; in the orange, orange; in the yellow, luminous objects may be seen through yellow, but less bright. In the green rays, them, as through plates of coloured glass. it is green; but from the great inaptitude Let one of these plates be interposed perof vermilion to reflect green light, it ap- pendicularly between the eye and a candle, pears dark and dull ; still more so in the the latter will be seen with equal distinctblue; and in the indigo and violet it is ness in every position of the axis of the almost completely black. On the other plate with respect to the horizon; and if hand, a piece of dark blue paper, of Prus- the plate be turned round on its own plane, sian blue, in the indigo rays has an extra- no change will be perceived in the image ordinary richness and depth of colour. of the candle. Now, holding this first In the green its hue is green, but much plate in a fixed position, (with its axis verless intense ; while in the red rays, it is tical, for instance,) let a second be interalmost entirely black.
posed between it and the eye, and turned In the above experiments, to make the round slowly in its own plane, and a very analysis complete, the beam of light to be remarkable phenomenon will be analyzed must be very small, and the prism The candle will appear and disappear as free from striæ or veins as possible, but alternately at every quarter revolution of as it is diffiult to procure prisms free from the plate, passing through all gradations these imperfections, the best way is to of brightness, from a maximum down to
a total, or almost total, evanescence, and certain positions of the plate; and ready thén increasing again by the same degrees transmission in others, at right angles to the as it diminished before. Now, it is evident former. that the light which has passed through 2. Incapability of being reflected by the first plate, has acquired, in so doing, polished transparent media, at certain ana property totally distinct from those of the gles of incidence, and in certain positions original light of the candle. The latter of the plane of incidence. would have penetrated the second plate 3. Incapability of undergoing division equally well in all its positions; the former into two equal pencils by double refracis incapable altogether of penetrating it in tion, in positions of the doubly-refracting some positions, while in others it passes bodies, in which a ray of ordinary light through readily, and these positions corre- would be so divided. spond to certain sides which the ray bas We have not room to describe the ex. acquired, and which are parallel and per- periments by means of which the phenopendicular respectively to the axis of the mena, above alluded to, are performed and first plate. Moreover, these sides, once explained ; but it may be necessary to reacquired, are retained by the ray in all its mark, that the characters of polarized light future course, (provided it be not again are all of the negative kind, and consist in otherwise modified by contact with other denying to it properties which ordinary bodies,) for it matters not how great the light possesses, and that they are such as affect distance between the two plates, whether the intensity of the ray, not its direction. they be in contact, or many inches, feet, or Thus, the direction which a polarized ray yards asunder, not the least variation is will take, under any circumstances of the perceived in the phenomenon in question. action of media, is never different from If the position of the first plate be shifted, what an unpolarized ray might take, and the sides of the transmitted ray shift with from what a portion of it, at least, actually it, through an equal angle, and the second does. For instance, when an unpolarized will no longer extinguish it in the position ray is separated by double refraction into it at first did, but must be brought into a two equal pencils, a polarized ray will position removed therefrom by an angle be divided into two unequal ones, one of equal to that through which the first plate which may even be altogether evanescent, has been made to revolve.
but their directions are precisely the same But it is not only by such means that the as those of the pencils into which the un. polarization of a pencil of light may be polarized ray is divided. Hence, it may effected, nor is this the only character be laid down as a general principle, that which distinguishes polarized from ordinary the direction taken by a polarized ray, or light. It may be as well, therefore, briefly by the parts into which it may be divided to mention the principal means by which by any reflexions, refractions, or other mothe polarization of light may be performed, difying causes, may always be determined and the characters which are invariably by the same rules as apply to unpolarized found to co-exist in a ray when polarized. light; but that the relative intensities of
The polarization of light may be effected, these portions differ from those of similar
1. By reflexion, at a proper angle from portions of unpolarized light, according to the surfaces of transparent media.
certain laws, which it is the business of the 2. By transmission through a regularly philosopher to ascertain. crystallized medium, possessed of the From the foregoing observations and property of double refraction.
experiments, the following facts, relative to 3. By transmission through transparent, the nature and properties of light, may be uncrystallized plates, in sufficient numbers, considered as established. and at proper angles.
1. Light has never been found collected 4. By transmission through a variety of in separate masses, but variously manifests bodies which have an approach to a lami- its existence in several bodies. nated structure, and an imperfect state of 2. Light possesses the property of ex. crystallization, such as agate, mother-of- citing in us the sensation of vision, by pearl, &c.
an illuminated object to the The characters which are invariably found eye. to co-exist in a polarized ray, and by which 3. The motion of light is progressive, it may be most easily recognized as polar- being known to occupy about seven and
a half minutes in moving from the sun to 1. Incapability of being transmitted by a the earth. plate of tourmaline, as above described, 4. Its progress may be stopped by the when incident perpendicularly on it, in interposition of an opaque body, and the
BRITISH ASSOCIATION FOR THE ADVANCE
132 BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. shadow or obseuration produced, proves 16. The different sorts of rays, as se. that light, in a uniform medium, moves in parated by the prism, have different destraight lines.
grees of illuminating power ; that is, a small 4. It enters into, and passes through, object will be more illuminated by a ray of certain substances, hence called transpa- one colour than by that of another.' rent; and, when it falls obliquely on them, The effects of light upon vegetation, solar it is bent or refracted at the surface. phosphori, and the various chemical changes
6. A large portion of light is reflected at it effects in bodies, will fall more properly the surfaces of the bodies on which it falls, under the head of chemistry, and may be especially when the surfaces are smooth and made the subject of another paper. polished; the rays being in this case copiously returned with great regularity, the incident and reflected ray making equal angles with the perpendicular to the surface.
MENT OF SCIENCE, HELD AT YORK, SE7. By means of refraction, or reflexion, at curved surfaces, a multitude of rays can
COND DAY, SEPTEMBER 27, 1831. be collected into a small space, or focus, producing there a strong light, and ex
(Continued from p. 34.) citing an intense heat in the substances About three hundred gentlemen assembled placed in the focus of the rays.
this morning in the theatre of the Museum. 8. Some bodies, on which the light falls, About half-past twelve o'clock, Lord Milseem to absorb a considerable part of it, so ton, the president of the Yorkshire Philothat it disappears.
sophical Society, was called to the chair. 9. All bodies, more or less, disperse in His lordship, in a very eloquent speech, all directions .some part of the light falling eulogized the Yorkshire Philosophical Soon them.
ciety, on account of its indefatigable in10. When the light falls on a crystallized dustry in promoting the interests of science; body, whose primitive form is not a cube, and stated his anticipations of the useful or octahedron, its rays are divided at the results of a national association of scientific surface, and pass through the crystal in two different directions; and the unusually re- The Rev. Wm. Vernon Harcourt, then fracted ray acquires peculiar properties. rose, and, with much eloquence, submitted
11. When a ray of light falls on the a plan for establishing a system of meetings polished surfaces of transparent bodies, like the present; and stated the grounds at a certain angle, different for different on which it was proposed. This meeting bodies, the reflected ray acquires properties owed its origin to some distinguished cultianalogous to the ray unusually refracted by vators of science; who thought, that great a crystal.
benefits would result to science in Great 12. When a small beam of light passes Britain, if meetings, similar to those on the through a triangular prism, it is divided Continent, were established among us. into parts by unequal refraction, exhibiting The council of the Yorkshire Philosophical on a screen a figure, or spectrum, contain- Society fully concurred in the great iming seven distinct classes of colours : that portance of the object, and, finding that which is least refracted, occupying the ex- many persons, eminent for literature and tremity nearest to the direction of the ori- science, entertained the same sentiments, ginal beam, is red; and the other extre- it was agreed to hold the first meeting in mity, or the light most refracted, is violet; the ancient city of York. The council had the order of the colours being red, orange, adjusted regulations and a plan upon which yellow, green, blue, indigo, and violet. a permanent establishment might be found
13. Each of these colours is permanent; ed; but, before detailing these to the meetfor, any one of them being any how re- ing, he should request Mr. Phillips, one of flected, or refracted, always shews the same the secretaries, to state what arrangements colour.
had been made by the committee, for this 14. Two or more of these colours being meeting, and also to read the letters which mixed, by being refracted to the same place had been received, in answer to invitatio on a screen, give a colour different from the Mr. Phillips then read, from the miprimitive ones.
nutes of the Yorkshire Philosophical Society, 15. All the colours of the spectrum, the proceedings which had been adopted; being so refracted as to fall on the same the first step was, the appointment of a place on a screen, give there the appear- committee, to make arrangements for the ance of a white speck, like that of the ori- reception of strangers coming to the present ginal beam.
meeting. The committee also addressed