Page:The American Cyclopædia (1879) Volume X.djvu/450

 444 LIGHT portions, each portion becoming, according to the principle of Huygens, the centre of a sys- tem of partial waves ; a theory whose correct- ness was demonstrated by the agreement which was found to exist between the calculation of the resultant of all the forces, according to the mathematical laws of interference and the in- tensities of light in the dark and light bands. Diffraction Spectrum.. If a piece of glass is ruled with very fine parallel lines, which may be done by a dividing engine and a dia- mond point, at the rate of from 1,000 to 5,000 or 6,000 to the inch, and it be looked through in the direction of a slit parallel with the gra- ting, a number of spectra will be seen which will be so pure in color as to exhibit several of Fraunhofer's lines. The spectra may be viewed with advantage by employing a tele- scope ; or an image of the slit, which must be highly illuminated, may be thrown upon a screen by a convex lens. Diffraction spectra are of great use in furnishing a uniform stan- dard of reference in the comparison of spectra, and as affording the most accurate method of determining the wave lengths of the different elementary rays of light. Fraunhofer,was the first to employ diffraction spectra. The first were made with fine wire stretched between the threads of screws about -gfa of an inch apart. This was comparatively a coarse gra- ting, but he soon ruled lines on glass much closer, and others have since executed them with great nicety. Mr. Rutherfurd of New York has ruled 12,960 lines to the inch, but finds that for practical purposes about half this number is preferable. Colors of Thin Plates; Newton's Rings. The phenomena known un- der these names were first examined by Boyle and Hooke, but were afterward more com- pletely investigated by Newton. They are ob- served in soap bubbles, plates of mica and selenite, and other crystals, in thin plates or films of glass or other transparent substances, or in the films of air held between two trans- parent plates. Any arrangement by means of which a ray of light may be reflected from two adjacent surfaces, or transmitted through two plates in such a manner as to produce in- terference of the rays, will exhibit the colors of thin plates. If the mouth of a small cylin- drical vessel is dipped into water made viscid with soap, a film will remain across it after its removal, which will exhibit the phenomena of these colors. Holding the film in a vertical position, it will at first, if thick enough, appear white ; but growing thinner by evaporation and descent of particles, it will soon present, commencing at the top, a brilliant play of iris- colored bands, arranged horizontally. After a while the top of the film loses its color and appears black, and growing thinner bursts. If a drop of oil or of spirits of turpentine is spread over the surface of water, a film of the proper thickness soon forms, which presents the same play of iris colors as the soap bubble. A fine- ly grooved surface, which has the power to re- flect the rays of light in such a way as to pro- duce interference, will also exhibit the same appearance. Newton examined the subject experimentally by placing a glass plate having a spherically convex surface of great focal length upon a plane glass, and applying a cer- tain degree of pressure. When the system is held toward the light a series of colored rings is observed, whose dimensions change with the amount of pressure. On looking atten- tively it will be observed that at the centre there is a circle of uniform color. If pressure is increased, this central circle dilates and at last forms a ring, and a new circle of a differ- ent color springs from its centre. This will in turn dilate and form a new ring, while an- other new circle will form within it, till at last a black spot appears at the centre of the system. After this no further pressure will produce a new circle, because we have now obtained a plate of air so thin as to be incapa- ble of reflecting light. Newton traced seven colored rings around this central spot, which are called the first, second, third, &c., order. Each order, when white light is used, contains all the primary colors ; thus, the red of the third order is the red in the third ring from the central black spot. The whole system of colors is called Newton's scale, and the princi- pal laws of the phenomena are : 1. In homo- geneous light the rings are alternately bright and black ; the thickness corresponding to the bright rays of succeeding orders being as the odd numbers of the natural series, and those corresponding to the black rings as the inter- mediate even numbers. 2. The thickness cor- responding to the ring of any given order varies with the color of the light, being great- est in the red light, least in the violet, and of intermediate magnitude in light of intermediate refrangibility. 3. The thickness correspond- ing to any given ring varies with the obliquity of the incident light, being very nearly pro- portional to the secant of the angle of inci- dence. 4. The thickness varies with the sub- stance of the reflecting plates, and in the in- verse ratio of its refractive index. To explain the effect of interference in producing the ring, let us consider a section of fig. 12. Let A B represent the convex plate, and D E the plane plate. There will be a film of air between them, whose thickness increases from the point of contact, C, in proportion to the H G F C F G H E FIG. 12. Explanation of Newton's Eings. square of the distance. If a beam of light is received upon either surface, a portion will be reflected by the surface of the film of air, and another portion by the surface of the glass be- yond it. There will thus be formed two sys- tems of waves intersecting each other, and in-