FIXED LINES IN THE SPECTRUM. 65 largest occupy a space from 5" to 10" in breadth. Sometimes they occur in well-defined lines, and at other times in groups; and in all spectra formed from solar light, they preserve the same order and intensity, and the same relative position to the coloured spaces, whatever be the nature of the prism by which they are produced. Hence these lines mark fixed points, by which the relative dispersive powers of different media may be ascertained with a degree of accuracy hitherto unknown in this branch of science. In the light of the fixed stars, and in that of artificial flames, a different system of lines is produced; and this system remains unaltered, whatever be the nature of the prism by which the spectrum is formed. The most important fixed lines in the spectrum formed by light emitted from the sun, whether it is reflected from the sky, the clouds, or the moon, may be easily seen by looking at a narrow slit in the window shutter of a dark room, through a hollow prism formed of plates of parallel glass, and filled with any fluid of a considerable dispersive power. The slit should not greatly exceed the twentieth of an inch, and the eye should look through the thinnest edge of the prism where there is the least thickness of fluid. These lines I have found to be the boundaries of spaces within which the rays have particular affinities for particular bodies.* • It would be out of place to enter farther into this (ubject here, and I leave it as left by Brewster. The study of these lines has become in recent years a science of itself, and brought astronomy into closer union with chemistry than could hare been thought possible, by the discoveries it has led to concerning the chemical constitution not only of the sun, but of many of the still more distant bodies of the universe.—Editor. CHAPTER VI. Colours of thin Plates first studied by Boyle and Hookc— Newton determines the Law of their Production—His Theory of Fits of easy Reflexion and Transmission— Colours of Thick Plates. In examining the nature and origin of colours as the component parts of white light, the attention of Newton was directed to the curious subject of the colours of thin plates, and to its application to explain the colours of natural bodies. His earliest researches on this subject were communicated, in his Discourse on Light and Colours, to the Royal Society, on the 9th of December, 1675,* and were read at subsequent meetings of that body. This discourse contained fuller details respecting the composition and decomposition of light than he had given in his letter to Oldenburg, and was concluded with nine propositions, showing how the colours of thin transparent plates stand related to those of all natural bodies. The colours of thin plates seem to have been first observed by Mr. Boyle. Dr. Hooke afterwards studied them with some care, and gave a correct account of the leading phenomena, as exhibited in the coloured rings upon soap bubbles, and between plates of glass pressed together. He "A little before this time Newton appears to have been occupied with a subject very different from his more usual pursuits—the planting of fruit trees for the manufacture of ddor. Our author, in the enlarged edition of this work, published a curious lottor, dated September S, 1676, too long to insert here, written by Newton to Oldenburg on this subject, which he had found amongst his papers.—Ekitor. recognised that the colour depended upon some certain thickness of the transparent plate; but he acknowledges that he had attempted in vain to discover the relation between the thickness of the plate and the colour which it produced. Dr. Ilooke succeeded in splitting a mineral substance called mica, into films of such extreme thinness as to give brilliant colours. One plate, for example, gave a yellow colour, another a blue colour, and the two together a deep purple; but, as plates which produced those colours were always less than the twelve-thousandth part of an inch thick, it was quite impracticable, by any contrivance yet discovered, to measure their thickness, and determine the law according to which the colour varied with the thickness of the film. Newton surmounted this difficulty by laying a double convex lens, the radius of curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in this way he obtained a plate of air or of space varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface, to a considerable thickness at the circumference of the lens. When light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glass gave different colours; so that the point whera the two object-glasses touched one another was the centre of a number of concentric coloured rings. Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena. In order to understand how he proceeded, let CED be the convex surface of the one object-glass, and AEB the flat surface of the other. Let them touch at the point E, and let homogeneous red rays fall upon them, as shown in ahe figure. At the point of contact E, where the plate of air is inconceivably thin, not a single ray of the pencil RE is reflected. The light is wholly transmitted, and, consequently, to an eye above E there will appear at E a black spot. At a, where the plate of air is thicker, the red light ra is reflected in the direction aa'; and as the air has the same thickness in a circle round the point E, the eye above E at a will see next the black spot E a ring of red b'ght. At m, where the thickness of the air is a little greater than at a, the light r m is all transmitted as at E, and not a single ray suffers reflexion, so that to an eye above E at m there will be seen without the red ring a a dark ring m. In like manner, at greater thicknesses of the plate of air, there is a succession of red and dark rings, diminishing in breadth, as shown in the diagram. When the same experiment was repeated in orange, yellow, green, blue, indigo, and violet light, the very same phenomenon was observed; with this difference only, that the rings were largest in red light, and smallest in violet light, and had intermediate magnitudes in the intermediate colours. If the observer now places his eye below E, so as to see the transmitted rays, he will observe a set of rings as * ■ COLOURS OF THIN PLATES. 67 before, but they will have a bright spot in their centre at £, and the luminous rings will now correspond with those which were dark when seen by reflexion, as will be readily understood from inspecting the preceding diagram. When the object-glasses are illuminated by white light, the seven systems of rings, formed by all the seven colours which compose white light, will now be seen at once. Had the rings in each colour been all of the same diameter, they would all have formed brilliant white rings, separated by dark intervals; but, as they have all different diameters, they will overlap one another, producing rings of various colours by their mixture. These colours, reckoning from the centre E, are as follows:— 1st Order. Black, blue, white, yellow, orange, red. 2d Order. Violet, blue, green, yellow, orange, red. 3rd Order. Purple, blue, green, yellow, red, bluish-red. 4th Order. Bluish-green, green, yellowish-green, red. 5th Order. Greenish-blue, red. 6th Order. Greenish-blue, red. By accurate measurements, Sir Isaac found that the thicknesses of air at which the most luminous parte of the first rings were produced, were in parts of an inch lTBOOO' 17B30 0 0' Itb'ooo' mVOTf' MtftjOO' 1 TsiOO' TM the medium or the substance of the thin plate is water, as in the case of the soap bubble, which produces beautiful colours according to its different degrees of thinness, the thicknesses at which the most luminous parts of the ring appear are produced at t^vjt °f tne thickness at which they are produced in air, and in the case of glass or mica at i-^-g of that thickness, the numbers 1'336, 1-525, expressing the ratio of the sines of the angles of incidence and refraction in the substances which produce the colours. From the phenomena thus briefly described, Sir Isaac Newton deduced that ingenious, though hypothetical, pro |