A much easier, though less simple, method of exhibiting interference phenomena is the fol lowing: If a hole, less than of an inch in diameter, be pierced in a piece of paper, a distant bright point, such as an arc light or the bright point of a mercury thermometer-bulb in sunshine, will appear to an eye looking through the hole as a disc surrounded by one or more concentric rings. If now a second similar hole be made within less than of an inch from the other and the experiment be repeated, the disc will appear brighter and traversed by a series of dark lines perpendicular to the di rection of a line joining the two holes. These dark lines mark the regions of complete inter ference and are exactly like the dark bands produced by Fresnel's method. The fact that the point-source of light appears as a disc of appreciable magnitude is also explicable by the fact that light is a wave-motion. Broadly stated the condition is this: if light consists of waves, we ought not to expect that the laws of reflection and of refraction will hold unmodi fied when the acting surfaces are no longer large with respect to the length of a light-wave. These deviations from the laws are called dif fraction.
Since most of the light with which we ordi narily experiment is composite, that is, made up of a combination of many different wave lengths, the places of complete destruction of illumination differ for differing wave-lengths. Such phenomena are, therefore, generally, some times splendidly, colored. Familiar cases of simple interference are presented by thin re flecting plates, such as soap-films, rifts in trans parent media, as in precious opal, etc. In these cases we have the two sources produced by re flection from the two sides of the plates, and the phenomena aregenerally known as the colors of thin plates, first studied by Newton.
Diffraction phenomena are even more com monly seen. The peculiar lustre of satin spar, of the gems known as cat's eye and of the star sapphire find their explanation here. The iri descence of mother-of-pearl and of certain feathers, and the brilliant-colored pattern seen when an electric arc light is viewed through a fine and uniform fabric, like silk or the web of many feathers, are diffraction phenomena of greater regularity than the former group.
Optical Phenomena of the Atmosphere. — Of the many optical phenomena which belong either to the air as a transparent body or be cause of foreign bodies temporarily suspended in it, mirages, coronas, rainbows and halos are among the most striking. The refractive power of the air increases with its density, and, as this is increased both by pressure and by lowering of temperature, there results varying effects of re fraction. With the normal condition of the air,
in which the density increases in a geometrical ratio as we rise above the surface of the earth, the only very obvious effect is to prolong the length of the day to twice the time which it re quires the sun to sink at the horizon by its own diameter. Atmospheric reflection is well illus trated by twilight; this reflection takes place partly from the air itself and partly from minute, solid particles in the atmosphere. It commonly remains visible until the sun has sunk 18 degrees below .the horizon, which indicates that at a height of 50 miles above the earth's surface the air is still dense enough to reflect an appreciable amount of light. Atmospheric dis persion is also present, but is not sufficiently marked to be detected by the unaided eye, al though conspicuous enough with a telescope under favorable conditions. The scintillation of the stars, however, is a direct effect of atmospheric dispersion.
In the not infrequent cases when the air de parts widely from the normal law of continu ous decrease of temperature with increasing height above the surface of the earth, the paths of the light waves which are nearly horizontal may change the direction of curvature between the object and the observer, in other words, may have points of inflection. In low latitudes the prevailing condition, when such abnormal re fraction may be observed, corresponds to a lower layer of air at a higher temperature than that inunediately above. In such cases distant objects near the horizon appear lifted above their real positions and portions of these much elongated vertically. A further development shows inverted images, generally without much distortion, underneath the raised images. Over sun-heated plains the inverted image is that of a portion of the sky, whence the effect of a sheet of water between the observer and the horizon. In high latitudes it is sometimes pos sible to find the condition of a layer of much cooler air in contact with sheets of ice or of cold water, in which case the inverted image is seen above the erect image. These phenomena are known under the name of mirage.
When the atmosphere is not quite clear one may sometimes see colored circles concentric with the sun or moon, generally not more than four or five times the diameter of the sun and invariably having the inner edge blue. Such circles are called coronas. They are diffraction phenomena produced by very small spherical drops of water suspended in the air, and their diameters are in an inverse ratio to the diameter of the drops to which they are due. The only necessary conditions for well-developed coronas are smallness and general uniformity of size in the drops.