10. The fluorescent rays: which are either a pure silvery blue or a delicate green.
Newton regarded the spectrum as consisting of seven colours of definite and unvarying refrangibility. Brewster and others appear to have detected a great diffusion of the colours over the spectrum, and regard white light as consisting only of three rays, which in the prismatic images overlap each other; and from these—red, yellow, and blue—all the others can be formed by combination in varying proportions. 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. We have already examined the heating power found in these coloured bands, which, although shown to be in a remarkable manner in constant agreement with the colour of a particular ray, is not directly connected with it; that is, not as the effect of a cause, or the contrary. The chemical action of the solar rays, to which from its important bearings we shall devote a separate chapter, has, in like manner with heat, been confounded with the sun’s luminous power; but although associated with light and heat, and modified by their presence, it must be distinguished from them.
We find the maximum of heat at one end of the spectrum, and that of chemical excitation at the other—luminous power observing a mean point between them. Without doubt we have these powers acting reciprocally, modifying all the phenomena of each other, and thus giving rise to the difficulties which beset the inquirer on every side.
We have beautiful natural illustrations of luminous refraction in the rainbow and in the halo: in both cases the rays of light being separated by the refractive power of the falling rain drop, or the vesicles which form the moisture constituting a fog. In the simple toy of the child—the soap-bubble floating upon the air—the philosopher finds subjects for his contemplation; and from the unrivalled play of colours which he discovers in that attenuated film, he learns that the varying thicknesses of surfaces influence, in a most remarkable manner, the colours of the sunbeam. Films of oil floating upon water present similar appearances; and the colours developed in tempering steel are due entirely to the thickness of the oxidized surface produced by heat. There have lately been introduced some beautiful specimens of paper rendered richly iridescent by the following process:—A solution of a gum resin in chloroform is floated upon water, where it forms a film giving all the colours of Newton’s rings. A sheet of paper which has been previously sunk in the water is carefully lifted, and the film thus removed adheres with great firmness to the paper, and produces this rich and curious play of colour. The rich tints upon mother-of-pearl, in the feathers of many birds, the rings seen in the cracks of rock-crystal, or between the unequal faces of two pieces of glass, and produced by many chemical and indeed mechanical operations—are all owing to the same cause;—the refraction of the luminous pencil by the condition of the film or surface. If we take one of those steel ornaments which are formed by being covered with an immense number of fine lines, it will be evident that these striæ present many different angles of reflection, and that, consequently, the rays thrown back will, at some point or another, have a tendency to cross each other. The result of this is, that the quantity of light is augmented at some points of intersection, and annihilated at others.[97] Out of the investigation of the phenomena of diffraction, of the effects of thin and thick plates upon light, and the results of interference, has arisen the discovery of one of the most remarkable conditions within the range of physical science.
Two bright lights may be made to produce darkness.—If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. If now we so arrange the points of radiation, that the systems of luminous waves proceed irregularly, and that one arrives at the screen half an undulation before the other, the one in elevation falling into the depression of the other, a mutual annihilation is the consequence. This fact, paradoxical as it may appear, was broadly stated by Grimaldi, in the description of his experiments on the inflection of light, and has been observed by many others. The vibratory hypothesis, seizing upon the analogy presented by two systems of waves in water, explains this plausibly, and many similar phenomena of what is called the interference of light; but still upon examination it does not appear that the explanation is quite free from objection.[98]
Another theory, not altogether new to us, it being indicated in Mayer’s hypothesis of three primary colours (1775), and to be found as a problem in some of the Encyclopædias of the last century, has been put forth, in a very original manner, by that master-mind of intellectual Germany, Goethe; and from the very comprehensive views which this poet-philosopher has taken of both animal and vegetable physiology (views which have been adopted by some of the first naturalists of Europe), we are bound to receive his theory of colours with every respect and attention.
Goethe regards colour as the “thinning” of light; for example, by obstructing a portion of white light, yellow is produced; by reducing it still farther, red is supposed to result; and by yet farther retarding the free passage of the beam, we procure a blue colour, which is the next remove from blackness, or the absence of light. There is truth in this; it bears about it a simplicity which will satisfy many minds; by it many of the phenomena of colour may be explained: but it is insufficient for any interpretation of several of those laws to which the other theories do give us some insight.
Newton may have allowed himself to be misled by the analogy presented between the seven rays of the spectrum and the notes in an octave. The mystic number, seven, may have clung like a fibre of the web of superstition to the cloak of the great philosopher; but the attack made by Goethe upon the Newtonian philosophy betrays the melancholy fact of his being diseased with the lamentable weakness of too many exalted minds—an overweening self-esteem.
The polarization of light, as it has been unfortunately called—unfortunately, as conveying an idea of determinate and different points or poles, which only exists in hypothetical analogy—presents to us a class of phenomena which promise to unclose the mysterious doors of the molecular constitution of bodies.
This remarkable condition, as produced by the reflection of light from glass at a particular angle, was first observed by Malus, in 1808,[99] when amusing himself by looking at the beams of the setting sun, reflected from the windows of the Luxembourg Palace through a double refracting prism. He observed that when the prism was in one position, the windows with their golden rays were visible; but that turned round a quarter of a circle from that position, the reflected rays disappeared although the windows were still seen.