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body in different states of density. Thus the apparent crookedness of a stick placed obliquely in water; the difficulty of hitting. a body, as a fish, in water, when we take an oblique aim; the deception experienced in estimating the depth of water, except when viewed perpendicularly; and the altered position of a body (as a piece of money) contained in a basin, when viewed obliquely, first when the basin contains no water, and afterwards when water has been put in—these, and many other phenomena, result from the greater refractive powers of water than of air, and the consequent change of direction which the luminous rays suffer when passing from one medium to the other. Again, the tremulous motions of bodies, when viewed through an ascending current of heated air, and by which an excise-officer is said to have, on one occasion, discovered a subterranean still in the Highlands of Scotland, result from-the unequal refracting power of air in different states of density.
6. Dispersion.—If a ray of white light be made to traverse a refracting medium, or, in other words, to suffer refraction, it is found to have undergone a remarkable change—it is no longer perfectly white, but more or less coloured. It is assumed, therefore, that white light is made up of coloured lights, and that these, being unequally refrangible, are separated, or, in optical language, are dispersed. In this way, seven colours are obtained, viz. violet, indigo, blue, green, yellow, orange,' and red. These are usually procured by a triangular piece of glass, called a. prism—the seven colours constituting the prismatic or solar spectrum. This mode of producing colours from white light is called the decomposition, the analysis, or the dispersion of light. If we allow the oxyhydrogen lime-light to pass out of the lantern through a slit, and receive it on a prism, the spectrum may be thrown on the cieling of the lecture-room, or on the screen before us.
To persons unacquainted with philosophical investigations, few facts seem more astonishing, and even improbable, than that of white light being compounded of differently coloured lights. I shall, therefore, dwell for a few minutes on this topic.
Every one is familiar with the fact, that, by mixture, colours are altered. Thus blue and yellow form green; red and yellow form orange; while blue, with different proportions of red, yields indigo or violet.
You will, therefore, readily believe, that of the seven prismatic colours into which the prism decomposes white light, three only may be primitive, and four compounded.
If the seven prismatic colours be rudely printed on a circular disk of card, and then be made to rotate rapidly, the union of these colours on the retina gives us an impression of greyish-white.
If we paint the three supposed primitive colours, viz., red, yellow, and blue on a similar disk, and cause this to revolve, we also obtain an impression of greyish white.
These experiments, therefore, favour the notion that the sensation of white light depends on the simultaneous impression of differently-coloured lights on the retina; and, secondly, that three of the prismatic colours being capable of giving the sensation of white light, they probably are the primitive colours, the others being compounds. Hence, white light is called compound or heterogeneous light; while the three colours, red, yellow, and blue, are termed simple or homogeneous lights. Each of these may be termed a monochromatic light. Orange, green, indigo, and violet, on the other hand, are mixed colours.
It follows, from this view of the subject, that two colours (one of which must be a mixed colour) may by their union or mixture produce white light. Colours or tints which do this are called complementary.'
Red... and Green.
Yellow " Indigo.
Blue... " Orange.
White and black are also said to be complementary.
I shall now proceed to demonstrate the accuracy of these
positions. If I throw two beams of coloured light, one red,
the other green, on a screen, we see two circular disks of
coloured light, and by making them overlap, they produce white
light. A similar result (that is, the formation of white light) is also produced by the overlapping respectively of disks of indigo and yellow, and of blue and orange. These colours are obtained by a complicated process. The oxyhydrogen lime-light is refracted J by the condensers in this lantern Formation oi while Light. — then polarized — then doubly
refracted or depolarized by a thin film of selenite—then refracted by the two powers—then analysed by a double refracting prism. By this process, the nature of which will be fully explained hereafter, we have destroyed the yellow and the blue, leaving the red, of one beam—while the red only has been destroyed in the other beam, leaving the yellow and the blue (which by their mixture constitute green). If we then cross the two beams, the red and the green by their mixture form white light.
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 nm 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 ore the other coloars 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 paperf, 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 luvtmilor-groy. But in a more recent paper}, he appears to have satisfied himself thaI the colour is yellow. '• And if such," he adds, "rallior than lavender or dovo colour, should be the true colorific character of these ray», w»' minht almost bo led to believe (from the evident ren1,penrnuce. of vetlne.** minu'lod with blue in the violet rays) in w r0pot.lt Inn of I he piiinwiy lint* in their order,
* This notion wan thrown out liy hMttowof Uimyo, III IIIl Lectures on Light, di'llvornil Ml. Hip Minion I iwl II ill Ion, In Nnvpinlwi', I mi), t Philuaoiilui'nt Tfini*H.l(,m», IW IB4II, 0 !»• j AM, fur !»•«, m» luh, IW».
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.
Hypothetical Spectrum... J Yellow
- -i Indigo.
} Green. ^Blue..
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 produceHhe 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, Calolypes*,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 (lifli'rcnt 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, awl 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.