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series of brilliant fringes parallel to the edges, and separated from each other by dark intervals; the brightness of these fringes diminishes as they recede from the shadow; and the shadow itself is not quite dark, but is formed also of luminous and dark fringes all parallel to the edges of the lamina. Moreover the ground glass is not necessary to exhibit these fringes, for they are formed in the air and may be seen in it, either with the naked eye or by the assistance of a lens placed exactly on their direction. If then a lens be fixed to a firm stand which can be moved horizontally, by means of a screw, along a scale divided into equal parts, its axis may be brought successively opposite each bright and dark fringe; the position of one of these may be determined precisely by referring it to a fine thread stretched in front of the lens, and thus the intervals of the fringes may be measured, on the graduated scale, by the distance through which the lens is moved to set it opposite to each; this advantageous arrangement was devised by M. Fresnel, who made use of it to measure all the particulars of the phænomenon with extreme precision.
Now these particulars, as Dr. Young first announced, may be represented pretty exactly by supposing that the light which falls on the edges of the lamina, spreads over them radiating in all directions from those edges, and interferes both with itself and with the rays transmitted directly.
The first kind of interference forms the interior fringes; the light radiating from one edge interfering with that from the other, these two sets of rays are exactly in the same predicament as the two luminous reflected points in the experiment of the mirrors; thus also the disposition of the interior fringes both bright and dark, and the ratios of their intervals, are exactly similar. If you determine in your mind the series of points in space at which the same kind of interference takes place, at different distances behind the lamina, which gives the succession of the places at which the same fringe appears, you will find that those points are, to all appearance, on a straight line; and their intervals, when measured, are very exactly conformable to what the calculation of the interferences indicates.
As to the exterior fringes, they may be considered as formed by the interference of the light transmitted directly with that radiating from each edge; but we must here, as in the reflected rings, sup-
