Imagens das páginas
PDF
ePub

The position of the orange or green lights in the spectrum favours the notion of their being mixed colours; since the orange is placed between the red and the yellow, and the green between the yellow and the blue.

But the indigo and the violet being placed the most remote from the red, appear to present an obstacle to this notion. Dr. Brewster, who adopts the supposition of there being three primitive colours, supposes that the solar spectrum consists of three spectra of equal lengths, a red, a yellow, and a blue one; and that the position of the maximum intensities of these colours varies, while certain portions of each of the three colours form white light, by mixing with the other colours in the requisite proportions-the excess of colour giving the predominating tint to that part of the spectrum where it exists. Thus in the red part of the spectrum, there is an excess of red rays, in the yellow part of yellow rays, in the blue part of blue rays, in the violet part of both blue and red rays.

This view of the subject has never appeared to me satisfactory; and accordingly another and a more probable one presenting itself, I am inclined to adopt it, more especially as it is supported by some experiments recently made by Sir John Herschel. Suppose a repetition of the primitive colours of the Newtonian spectrum; the red of the second spectrum being partially superposed on the blue of the first spectrum. The extreme blue rays of the first spectrum being intermixed with the red rays of the second spectrum, will give the sensation of indigo and violet. But it may be asked, where are the other colours of the second spectrum? The reply is, that they are not visible to the eye. What evidence, then, it will be said, is there of the existence of invisible rays beyond the first or Newtonian spectrum? The evidence is twofold-first, the well-known chemical effects produced beyond the visible spectrum; and, secondly, Sir John Herschel's experiments before referred to. In his first paper+, in which he announces the extension of the visible prismatic spectrum, and the existence of a new prismatic colour beyond the violet; he states, that this colour appeared to his eyes as well as to those of a friend, to be lavender-grey. But in a more recent papert, he appears to have satisfied himself that the colour is yellow. "And if such," he adds, "rather than lavender or dove colour, should be the true colorific character of these rays, we might almost be led to believe (from the evident reappearance of redness mingled with blue in the violet rays) in a repetition of the primary tints in their order,

This notion was thrown out by Professor Grove, in his Lectures on Light, delivered at the London Institution, in November, 1842. + Philosophical Transactions, for 1840, p. 19.

Ibid, for 1842, pp. 195, 196.

beyond the Newtonian spectrum, and that if, by any concentration, rays still farther advanced in the chemical spectrum could be made to affect the eye with a sense of light and colour, that colour would be green, blue, &c., according to the augmented refrangibility."

The following diagram serves to illustrate this view:

Primitive Colours. Mixed Colours.

[blocks in formation]

There are many interesting topics connected with the spectrum, the details of which I feel precluded from entering into, inasmuch as these lectures are intended to illustrate the phenomena of polarized light. I must, therefore, content myself with briefly naming some of them. The first is the unequal refrangibility of the difrent-coloured rays; the red being the least, the violet the most refrangible. It is in virtue of this property that lenses and prisms produce the phenomena of dispersion or chromatic aberration. Newton thought that the size of the spectrum, or the dispersive power of the refractive medium, was proportional to its refractive power; and, therefore, that the refracting telescope could not be made achromatic. In this he was mistaken. Equal refractions do not produce equal dispersions. Two lenses made of different kinds of glass, as one of crown the other of flint glass, may be so ground that the dispersions shall neutralize each other, while their refractions, not being equal, cannot neutralize; consequently, an excess of one remains.

Not the least remarkable fact connected with the spectrum, is the existence of bands or fixed lines in it. They are commonly called Fraunhofer's lines of the spectrum. The best mode of seeing them is to examine the spectrum by a telescope *. The light (as of a lamp, or that produced by throwing the oxyhydrogen flame on lime) should be passed through a bottle filled with nitrous acid vapour before it falls on the prism, to produce the spectrum.

The illuminating, heating, magnetic, and chemical powers of the spectrum, I must pass over without further notice, as they have no direct connection with the immediate object of this course of lectures. I cannot resist, however, remarking that the

* An apparatus for the exhibition of these lines, lent by Messrs. Watkins and Hill, was exhibited to the meeting.

existence of a calorific, magnetic, and chemical influence beyond the confines of the coloured spectrum, is a fact of considerable importance in any enquiries which may be instituted into the nature of light. Moreover, the splendid and interesting pictures called Daguerreotypes, Calotypes*, Chrysotypes, and Ferrotypes, or Cyanotypes, now before me, produced by the chemical influence of light on gross or ponderable matter, show the high importance of investigations respecting the chemical powers of the spectrum.

7. Diffraction. When light passes near the edges of bodies, it suffers certain modifications, included by opticians under the denomination of inflection or diffraction. If an opake body be placed in a cone of light admitted into a dark chamber through a very small aperture, its shadow is larger than its geometric projection. Moreover, its shadow is bordered with fringes, and similar fringes are observed within the shadows of narrow bodies.

If the light be homogeneous or monochromatic, the fringes consist of dark and light spaces of the same colour, and are of different breadths, red yielding the broadest, violet the narrowest fringes, but in white light the fringes are prismatic or iris-coloured.

The iris fringes may be readily observed by looking through the slit, between the almost closed fingers, at a candle, placed at a distance of several yards. It may be seen still better by looking at the same luminous body through a feather, or through a fine wire gauze. I have before me Schwerd's very complete apparatus for examining the complicated and difficult phenomena of diffraction.

When I tell you that the immortal Newton failed to perceive the internal fringes, and that he left the subject altogether in an imperfect, unsatisfactory, and unfinished state, I need scarcely add, that the phenomena are very complicated, and their study exceedingly difficult.

8. Colours of thin plates, of films, and of grooved surfaces. A variety of curious and brilliant optical phenomena were attributed by Newton to what he called fits of transmission, and fits of reflection; but which Dr. Young and most subsequent writers ascribe to interference of light. I refer now to the phenomena of thin plates, of films, and of grooved surfaces.

Excessively thin plates of air, liquids, or solids, appear coloured when viewed by reflected and transmitted light; but the colour seen by reflection is complementary to that seen by transmission.

If the plate be of uniform thickness, the colour is uniform; but if the thickness varies, the colour also varies.

Some beautiful Calotype portraits, taken by Mr. Collen, of Somerset Street, Portman Square, miniature painter to the Queen, were exhibited to the meeting.

Very much thinner plates than those which present colours, do not reflect light, and when viewed in this position, appear black. But they still transmit light, and when viewed by transmitted light, appear white.

Wedge-shaped plates present a series of parallel bands or fringes of colour.

A plate having the form of a plano-concave lens, the thinnest part of the plate being in the centre, gives a series of concentric rings of brilliant colours. Those seen by reflected light, have a black spot in the centre, while the transmitted rings have a white spot in the centre.

These different phenomena of thin plates are brilliantly illustrated in the lecture-room by the oxyhydrogen lime-light, which, after passing through the condensers of the lantern, is polarized, then passed through films of selenite (of uniform thinness, or wedge-shaped plates, or plano-concave films) afterwards through the two lenses called the powers, and ultimately analysed by a plate of tourmaline, or a bundle of plates of thin glass. The nature of the changes will be explained hereafter.

The squares of the diameters of the reflected coloured rings are as the odd numbers, 1, 3, 5, 7, 9, &c.; while the squares of the diameters of the transmitted rings are as the even numbers, 0, 2, 4, 6, 8, 10, &c.

The brilliant colours, produced by thin plates of air between the laminæ of mica, of selenite, and of Iceland spar, and between plates of glass, are familiar illustrations of the colours caused by thin plates of a gaseous substance.

The colours caused by thin films of oil of turpentine or other essential oils, of alcohol or of water, and by soap-bubbles, are wellknown examples of the colours caused by thin plates of liquids.

The iridescent hues produced on copper or steel by heat, and which depend on the formation of a thin film of metallic oxide, are good illustrations of the colours caused by thin plates of solids. But the most brilliant are those caused by thin films of peroxide of lead, formed upon polished steel plates, by the electrolytic decomposition of acetate of lead. These splendid prismatic tints were discovered by Nobili*, and are commonly known as Nobili's colours or metallo-chromes. The mode of producing them has been described by my friend Mr. Gassiot, in a paper read before the Royal Society. If we place on the polished steel plate a card screen in which some device is cut out,very beautiful figures, having a splendid iridescent appearance, are produced.

In all the cases hitherto alluded to, I have supposed white or

*See Taylor's Scientific Memoirs, vol. i. part 1.

+ See the Proceedings of the Royal Society, for March, 1840; also Brande's Manual of Chemistry, 5th edit., p. 836.

compound light to be used; and then the colours are iridescent or prismatic. But if monochromatic or homogeneous light be employed, the rings are of a uniform tint cr colour, and are separated by obscure bands or rings. Red light yields the broadest, violet light the narrowest rings.

Minute particles, fibres, and grooved surfaces also produce prismatic or iridescent colours by white light. Thus, minute particles of condensed vapour, obtained by breathing on glass, give rise to this effect. A familiar illustration is to be found in the halos observed around the street-lamps, when viewed at night through a coach-window, on the glass of which vapour is deposited. In this case the colours are seen by transmission. Dr. Joseph Reade's beautiful instrument, called the Iriscope, brilliantly displays the colours produced by reflection from a plate covered with condensed vapour. It consists of a plate of highly-polished black glass, having its surface smeared with a solution of fine soap, and subsequently dried by rubbing it clean with a piece of chamois leather. If the surface, thus prepared, be breathed on, through a glass tube, the vapour is deposited in brilliant coloured rings. But as, in this mode of experimenting, the plate of vapour is thickest in the middle,and thinnest in the circumference, the rings have black circumferences instead of black centres.

Minute fibres of silk, wool, and of the spider's web, also present in sunshine a most vivid iridescence.

A very minutely grooved surface also presents a prismatic or iridescent appearance in white light. Of this mother-of-pearl is a familiar instance-as also opal. Micrometer scales frequently present the same appearances; and Barton's buttons and other iris ornaments owe their resplendence to the numerous minute grooves cut in the surface of the metal. If a beam of light from the oxyhydrogen apparatus be received on one of Barton's buttons, an iridescent image may be thrown on a screen several yards distant; thus furnishing a good lecture-room illustration of the colours of grooved surfaces.

9. Double Refraction.-When a pencil of light falls in certain directions on any crystals, which do not belong to the cubical system, it is split or divided into two other pencils, which diverge and follow different paths; and when their divergence is considerable, objects viewed through them appear doubled. The change thus effected on a ray of light is denominated double refraction. The substance which is commonly used to produce this effect is that variety of transparent crystallized carbonate of lime, called Iceland spar, or sometimes calcareous spar, or, for brevity, calc-spar. In every double refracting crystal there are, however, one or more directions in which double refraction does not take place. These are called axes of double refraction: they might with more propriety be termed axes of No double refraction.

« AnteriorContinuar »