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DIES

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mFFRACTION-GRATlNtl

was made, she cut the hide into small thongs and so surrounded a large piece of ground, on which rose the city of Carthage. Vergil represents Dido's death to have been caused by the desertion of JSneas, who left her and sailed to Italy.

Dies Irae (di'ez i're), the name of a celebrated Latin hymn, the title coming from the first line, Dies ir&, dies ilia. Its subject is the Last Judgment. It first appeared at Venice in 1250. No religious poem has been oftener translated; but no one of the hundred or more versions which have been made is equal to the original in poetic fire and religious fervor. This is Scott's rendering of the first stanza:

The day of wrath! that dreadful day. When heaven and earth shall pass away! What power shall be the sinner's stay? How shall he meet that dreadful day?

Diffrac'tion, a phenomenon which occurs when waves of any kind pass the edge of

an obstacle. The simplest ilustration of the phenomenon is perhaps the case of waves washing up against a break-water with an opening in it. As shown in the accompanying

figure, the wave does not pass through the small opening A undisturbed, but spreads out in all directions, giving rise to a series of more or less spherical waves. This spreading out is called diffraction, which is merely the Latin word for bending apart. The easiest waves with which to observe diffraction are light-waves. For this purpose it is only necessary to take a sewing-needle and prick a very small hole in a visiting card. Now hold this hole immediately in front of the eye and look through it at some small or narrow source of light, say the filament of an incandescent lamp. The filament will not appear so bright as without the card, but it will appear much wider, showing that the light-waves from any one point on the filament, in passing through the small hole (that is, in passing the edge of the obstacle) are spread out into a cone, as indicated in the accompanying figure, so that the source of light appears to subtend the angle included between the dotted lines. This phenomenon

B

The effect produced upon waves by a small opening in a breakwater

is known as the diffraction of light. Soundwaves behave^ in the same way. A beautiful case of diffraction, which may be observed almost any evening on a street illuminated by arc-lights, is the following; a twig of a tree, a lead-pencil or any opaque

object will give a shadow on a white card which you may hold in your hand. This shadow will be bordered by two or three lines alternately bright and dark. These bands are due to diffraction past the edge of the opaque obstacle. They were discovered more than 200 years ago by an Italian, and are called, after him, Gri-maldi's fringes. The explanation of them, which is too long for this brief sketch, was first given by Fresnel in the early part of the 19th century. See Preston's Theory of Light.

Diffrac'tion=Grating, an instrument for separating from one another light-waves of different lengths. A diffraction-grating is simply an instrument which, when placed in the path of a pencil of light, will impress a different direction upon each different color which is present in the pencil of light. A grating consists of a large series of very narrow, parallel apertures equidistant one from another. The light-waves, in passing through these apertures, are diffracted, and each wave-length is sent off in a different direction from every other wave-length. Just how this happens is top complicated for explanation in this brief sketch. Diffraction-gratings were devised by the distinguished German optician, Fraunhofer. He made them by winding fine wire about two parallel screws cut with fine threads. They are now made by ruling, with a diamond-point, a polished surface, such as glass or speculum metal. The unruled portion acts as an aperture for light, which is transmitted through the grating or reflected from its face. All the best gratings of the world have been made by the late Professor Henry A, Rowland of Johns Hopkins University. Most of his gratings are ruled on speculum-metal with 14,438 lines to the inch; but a few of them have been made with as high as 42,000 lines to the inch. The largest error in the position of any one of these lines is said to be less than one one-hundred-thcusandth of an inch. The first thing absolutely