Monochromatic light

“ With monochromatic light the rings would be simply bright and black—the bright rings occurring at those thicknesses of the spar which cause the rays to conspire; the black rings at those thicknesses which cause them to quench each other. Turning the analyzer 90° round, we obtain the complementary phenomena. The black cross gives place to a bright one, and every dark ring is supplanted also by a bright one (fig. 46) . Here, as elsewhere, the different lengths of the light- [Pg 146] waves give rise to iris-colours when white light is employed. ”

“ That natural colours are the result of white light, and not innate properties of the bodies themselves, is most conclusively shown by placing coloured bodies in monochromatic light of another kind, when they will appear either of the colour of that light, or, by absorbing it, become black; whereas, when placed in light of their own character, the intensity of colour is greatly increasing. ”

“ Monochromatic light is utilized in photo-micrography for the best rendition of detail. Light-waves have been utilized as standards of length because they are invariable and fundamental. Numerous other interesting adaptations of artificial light are in daily use. ”

“ Since a beam of common light can be resolved into plane polarized streams and these on re composition give a stream with properties indistinguishable from those of common light, whatever their relative retardation may be, it is natural to assume that an analytical representation of common light can be obtained in which no longitudinal vector occurs. On the other hand a stream of strictly monochromatic light with a polarization-vector that is entirely transversal must be (in general elliptically) polarized. Consequently it follows that common light cannot be absolutely monochromatic. ”

“ In the same manner the phænomena, when the incident light is circularly analysed by a mica plate inserted before the Iceland spar, are throughout similar to those before described. Instead of the homogeneous rays of the spectrum, we can of course also employ in these experiments a monochromatic lamp or absorption by coloured glasses. It is only when the light has been circularly polarized in one colour through a plate of definite thickness that we can determine, whether the difference of path of the two rays be or or undulation. ”

“ Although the two theories give laws of exactly the same form for monochromatic light in the limiting case, their deviations from this limit are in opposite directions, so that if the phenomena of optics differed in any marked degree from what we would have in the limiting case, it would be eiwy to find an experimentum crucis to decide between the two theories. ”

“ The glare of white light round the sun can be dispersed and weakened to any extent, by augmenting the number of prisms; while a monochromatic [Pg 205] light, mixed with this glare, and masked by it, would retain its intensity unenfeebled by dispersion. Upon this consideration has been founded a method of observation, applied independently by M. Janssen in India and by Mr. Lockyer in England, by which the monochromatic bands of the prominences are caused to obtain the mastery, and to appear in broad daylight. ”

“ It is somewhat singular that no substance has yet been found that will increase the refrangibility of the dark rays beyond the red end of the spectrum. There are many artificial flames which produce this dark light (if we may use such a paradoxical expression) in greater quantity than the sun, whose light is no doubt greatly deteriorated in this respect during its passage through the atmosphere. The substance of which the prism is made also greatly influences the length of the invisible portion of the spectrum. ”

“ This number represents the rate of vibrations in the red note of our prismatic scale. The rate of vibration increases throughout the scale until with the ultra-violet it reaches 61,000 to the inch. Here we step out once more into the unknown.
Yet color has no actual existence. It is only by courtesy that we can use the word. Nature is a monochrome save when there are living eyes to see it. The trees are not really green, nor are the flowers red and yellow and blue. Each object simply reflects rays of light which vibrate at a given rate of speed ”

“ In looking at the spectrum we can not discern the light or heat below the red color, because the waves are so slow. Ascending the gamut of color, the rapidity of the vibrations increases, until just beyond the violet it becomes so great that the eye can detect no color. ”

“ The spectrum, then, of a body is simply the light proceeding from it spread out by refraction [399] into a brilliant variegated band, passing from brownish-red through crimson, orange, yellow, green, and azure into dusky violet. The reason of this spreading-out or "dispersion" is that the various colours have different wave-lengths, and consequently meet with different degrees of retardation in traversing the denser medium of the prism. ”

“ In virtue of this property the blue and violet end of the spectrum is more stimulating to the eye than the red end when the general luminosity is low, whereas at high luminosities the red gains relatively in brightness until it becomes more stimulating than the blue. Unless therefore account is taken in some definite measurable manner of the absolute brightness, there must always be some uncertainty in the photometric comparison of the intensities of differently coloured sources of light. ”

“ And partly also from the nature of the refraction it self, for the vividness or strength of the two terminating colours, arising chiefly as we have seen, from the very great difference that is betwixt the outsides of those oblique undulations & the dark Rays circumambient, and that disparity betwixt the approximate Rays, decaying gradually: the further inward toward the middle of the luminous body they are remov'd, the more must the colour approach to a white or an undisturbed light. ”

“ The practical determination of the second constant is possible in a great many cases; it presents itself always in the shape of a rather troublesome photometric problem, capable of a more or less accurate solution. The relative brightness of the colors of the solar spectrum is one of the most interesting of these problems, as its solution would serve to give some idea of the relative brightness of the colors, which, taken together, constitute white light. ”

“ Like green, it is of a neutral character, but at the same time its quality is opposite to that of green. In Newtonian optics, which assumes colour to be derived from light only, this colour has naturally no existence. Yet in an optics which has learnt to reckon with both darkness and light as generators of colour, the complete spectrum phenomenon includes this colour equally with green. ”

“ If we throw the whole spectrum on the screen, and ask a person with normal vision to point out the brightest part, he will indicate the yellow, whilst a red-blind will say the green, and so on. This tells us that the various types of colour blind must see their spectrum colours with luminosity differing from that of the normal eye. The difference can be measured by causing both to express their sense of the brightness of the different parts of the spectrum in terms of white light, or of one another. Brightness and luminosity are here used synonymously. ”

“ Let any one look at a spectrum of fair88 length, or even at a correctly tinted painting of the solar spectrum, and note how utterly unrecognizable to ordinary vision is the difference of tint for even the twentieth part of the distance between medium green and medium yellow on one side or medium blue on the other, and he will recognize how utterly hopeless it would be to attempt to appreciate the change of colour due to the approach or recession of a luminous body shining with pure green light and moving at the tremendous rate of 100 miles per second. ”

“ The tones of nature appear to us therefore different, like those of the spectrum, and for the same reason. The colours vary with the intensity of light. There is no colour peculiar to any object, but only more or less rapid vibration of light upon its surface. The speed depends, as is demonstrated by optics, on the degree of the inclination of the rays which, according to their vertical or oblique direction, give different light and colour. ”

“ When it is remembered that in colouring a white surface with thin colours, the white materially qualifies the colours, it must be admitted that the experiment fails to support the assertion that the colours of the spectrum produce white. But there can be no difficulty in understanding that a ray of light undergoing refraction, becomes divided into minor rays, which differing in their degrees of refrangibility, vary also in the velocity of their vibrations, and produce the several sensations of colour. ”

“ Sunlight can, by means of the prism, be split into colored rays, any one of which we may isolate, and so obtain a certain colored light. Similarly we may obtain light of a desired color by the use of a colored glass which will stop out the rays not of the hue required. So that we may obtain violet light from the spectrum or by filtering sunlight through violet glass. ”

“ The darkest colours, in distance, will be most of all impregnated with the colour of the air. So will the strongest real or accidental shadows.
Colours and outline are best defined on objects placed out of the strong light of the sun, and its reflexes. In sunshine, both are operated on by refraction, which occasions that chaotic indistinctness so painful for the eye to dwell on long together.
Every body, on which light falls, reflects a part of it back again. Any thing red, held before a looking-glass, gives back a portion of its own colour with great vividness ”

“ This is due to no other cause than that which produces [Pg 112] the rainbow and the spectrum—the separation, or "dispersion" in their passage through a refracting medium, of the variously coloured rays composing a beam of white light. In an ordinary lens there is no common point of concentration; each colour has its own separate focus; and the resulting image, formed by the superposition of as many images as there are hues in the spectrum, is indefinitely terminated with a tinted border, eminently baffling to exactness of observation. ”

“ Where the transverse impulse, which is due to the varying degree of TrÃ1⁄4bung in the light-realm, is directed towards the latter's edge, the intermingling of the Dark-ingredient and the Light-ingredient, contained in that realm, is such that Dark follows Light along its already existing gradient, thereby diminishing steadily. Hence our visual ray, meeting conditions quite similar to those occurring when we look across the light-filled atmosphere into universal space, notifies us of the presence of the blue-violet colour-pole. ”

“ At the blue pole, the lightness-effect springs from the resistant medium through which we gaze, a medium under the influence of gravity, while the darkness is provided by the anti-gravity quality of cosmic space, which as a 'negative' resistance exercises a suction on the eye's inner light. At the yellow pole it is just the reverse. Here, the resistant medium brings about a darkening of our field of vision, while the lightness-effect springs from a direct meeting of the eye with light, and so with the suctional effect of negative density. ”

“ The introduction of colour as a source of distinction, is the only means of obtaining a sufficient number of varieties. Yet, in itself, it is an evil of no small magnitude. The effect being produced by interposing coloured media between the burner and the eye of the observer, much light is lost by the absorption of those rays which are retained in order to produce the desired appearance. Experiments have been made with almost every colour ”

“ Hence, if two lights of unequal illuminating powers shine upon the same surface at equal obliquities, and an opaque body be interposed between them and the illuminated surface, the two shadows produced must differ in blackness or intensity in the same degree. For the shadow formed by intercepting the greater light, will be illuminated by the smaller light only ”

“ The truth will probably be found to be, that the ordinary prismatic spectrum is a compound of two spectra:—that is, as we have the ordinary rainbow, and a supplementary bow, the colours of which are inverted, so the extraordinary may be somewhat masked by the intense light of the ordinary spectrum; and yet by overlapping produce the variations of colour in the rays. ”

“ There is the fact to remember in making these measures in the extreme red and the extreme violet, that the luminosities of the colours are so small that the illumination of the prism itself, by the white light falling on it, has to be taken into account, since it forms an appreciable portion of the patch of feeble colour. By placing a proper shade of blue or red glass in the front of the collimator slit this white light disappears or becomes negligible, and when the absorption of the coloured glass is known from measurement, we can get a very accurate measure of the extinction of these parts. ”

“ Light is not homogeneous, but decomposable by refraction, absorption, or reflection, into coloured rays of unequal refrangibility. A ray of white light, in passing through a glass prism, is entirely separated into the coloured rays forming the ‘prismatic spectrum,’ and when it passes through a lens, an analogous resolution into coloured rays still occurs, though not so readily observed, and that to an extent often incompatible with distinct vision. ”