The heavy door opened with a grating sound.
Rutherfurd introduced into common use the reflection grating, finding that speculum metal was less trying than glass to the diamond point, upon the permanence of which so much depends.
The grating iron door made him jump.
He has also shown how to rule a plane surface with lines so disposed that the grating shall of itself give well-focused spectra.
This is the ordinary formula for a reflecting plane grating, and it shows that the spectra are formed in the usual directions.
There is also some uncertainty as to the actual temperature of the grating when in use.
In spite of the many improvements introduced by Rowland and of the care with which his observations were made, recent workers have come to the conclusion that .errors of unexpected amount have crept into his measurements of wave-lengths, and there is even a disposition to discard the grating altogether for fundamental work in favour of the so-called " interference methods," as developed by A.
The earliest is that of Quincke, who coated a glass grating with a chemical silver deposit, subsequently thickened with copper in an electrolytic bath.
It was formerly the custom in our village, when a poor debtor came out of jail, for his acquaintances to salute him, looking through their fingers, which were crossed to represent the grating of a jail window, "How do ye do?"
To eliminate the light returned from the hinder surface of an engraved grating, he covered it with a black varnish.
The light stopped by the opaque parts of the grating, together with that distributed in the central image and lateral spectra, ought to make up the brightness that would be found in the central image, were all the apertures transparent.
Let us suppose that the light is incident perpendicularly, and that the grating interval increases from the centre towards that edge which lies nearest to the spectrum under observation, and decreases towards the hinder edge.
V sin ?i, where a is the grating-interval and 43, the obliquity, the closeness of the grouping increasing with the number of intervals.
If x and y be co-ordinates in the plane of the wave-surface, the axis of y being parallel to the lines of the grating, and the origin corresponding to the centre of the beam, we may take as an approximate equation to the wave-surface -- -} z =+Bxy 2, +ax 3 13x2 2pp p y+-yxy2-?-Sy3+..
A disturbance limited to an infinitely thin slice of the medium, is supposed to fall upon a parallel grating, which again may With the best thickness so that in this case w=0 w = (2 w w= 0 be regarded as formed of infinitely thin wires, or infinitely narrow lines traced upon glass.
If we compare the spectrum produced by refraction in a glass prism with that of a diffraction grating, we find not only that the order of colours is reversed, but also that the same colours do not occupy corresponding lengths on the two spectra, the blue and violet being much more extended in the refraction spectrum.
If the sodium is only gently heated, so as to produce a comparatively rarefied vapour, and a grating spectroscope employed, the spectrum obtained is like that shown in fig.
The best object for examination is a grating of fine wires, about fifty to the inch, backed by a sodium flame.
One of these, of width equal, say, to one-tenth of an inch, is inserted in front of the object-glass, and the telescope, carefully focused all the while, is drawn gradually back from the grating until the lines are no longer seen.
In the case of a reflection grating the same method applies.
From (5) we see that, when the light falls perpendicularly upon a grating (0=o), there is no spectrum formed (the image corresponding to m=o not being counted as a spectrum), if the grating interval a or (a+d) is less than X.
Then the relative retardation of the extreme rays (corresponding to the edges A, B of the grating) is mnX.
It is evident that the waves from both halves of the grating are accelerated in an increasing degree, as we pass from the centre outFIG.
16) represent the surface of the grating, 0 being the centre of the FIG.
The grating at A and the eye-piece at 0 are rigidly attached to a bar AO, whose ends rest on carriages, moving on rails OQ, AQ at right angles to each other.
Mag., 1887) that the angular measurements present less difficulty than the comparison of the grating interval with the standard metre.
The grating would in any case retain its utility for the reference of new lines to standards otherwise fixed.
The metallic plate thus produced formed, when stripped from its support, a reflection grating reproducing many of the characteristics of the original.
The use of a grating is very convenient, for not only are there several spectra in view at the same time, but the dispersion can be varied continuously by sloping the grating.
In larger plant the upper ends of the sluices are often cut in rock or lined with stone blocks, the grating stopping the larger stones being known as a " grizzly."
And upwards in area, which are placed somewhat below the main sluice, and communicate with it above and below, the entry being protected by a grating so that only the finer material is admitted.
Cesareo; this is a marble altar richly decorated with mosaic in sculptured panels, and (below) two angels drawing back a curtain (all in marble) so as to expose the open grating of the confessio.
" it Polity was thought that in future it would be more consonant with the imperial dignity for the sovereign to remain concealed behind a grating where, unseen, he could hear all that was said.
Observing through a telescope with light perpendicularly incident, he showed that the position of any ray was dependent only upon the grating interval, viz.
In different gratings the lengths of the spectra and their distances from the axis were inversely proportional to the grating interval, while with a given grating the distances of the various spectra from the axis were as i, 2, 3, &c. To Fraunhofer we owe the first accurate measurements of wave-lengths, and the method of separating the overlapping spectra by a prism dispersing in the perpendicular direction.
The directions of the lateral spectra are such that the passage from one element of the grating to the corresponding point of the next implies a retardation of an integral number of wave-lengths.
If the grating be composed of alternate transparent and opaque parts, the question may be treated by means of the general integrals (§ 3) by merely limiting the integration to the transparent parts of the aperture.
The lateral (spectral) images occur in such directions that the projection of the element (a+d) of the grating upon them is an exact multiple of X.
The effect of each of the elements of the grating is then the same; and, unless this vanishes on account of a particular adjustment of the ratio a: d, the resultant amplitude becomes comparatively very great.
A+d If B denotes the brightness of the central image when the whole of the space occupied by the grating is transparent, we have Bo:B =a2:(a+d)2, and thus (2).
We conclude that, with a grating composed of transparent and opaque parts, the utmost light obtainable in any one spectrum is in the first, and there amounts to I/wr 2, or about 6, and that for this purpose W a and d must be equal.
In an engraved glass grating there is no opaque material present by which light could be absorbed, and the effect depends upon a difference of retardation in passing the alternate parts.
If it were possible to introduce at every part of the aperture of the grating an arbitrary retardation, all the light might be concentrated in any desired spectrum.
Michelson's ingenious echelon grating constitutes a realization in an unexpected manner of what was thought to be impracticable.
- [R.1 dicularly upon the grating; but the theory is easily extended.
When the grating is so situated that the angles of incidence and diffraction are equal.
Under these circumstances, if the material of the grating be completely transparent, the whole of the light must appear in the direct image, and the ruling is not perceptible.
According to our former standard, this gives the smallest difference of wave-lengths in a double line which can be just resolved; and we conclude that the resolving power of a grating depends only upon the total number of lines, and upon the order of the spectrum, without regard to any other considerations.
Let us take the case of a grating 1 in.
However this may be, it would always be possible, with the aid of a grating of given resolving power, to construct artificially from white light mixtures of slightly different wave-length whose resolution or otherwise would discriminate between powers inferior and superior to the given one.3 2 Compare also F.
3 The power of a grating to construct light of nearly definite wavelength is well illustrated by Young's comparison with the production of a musical note by reflection of a sudden sound from a row of palings.
The advantage of approximate bisection lies in the superior brilliancy of the surviving spectra; but in any case the compound grating may be considered to be perfect in the longer interval, and the definition is as good as if the bisection were accurate.
In the present application 4' is not necessarily equal to; but if P correspond to a line upon the grating, the difference of retardations for consecutive positions of P, so far as expressed by the term of the first order, will be equal to mX (m integral), and therefore without influence, provided v (sin 0-sin0') = nzX (11), where a denotes the constant interval between the planes containing the lines.
The parallelism, which is required to avoid aberrations, otherwise introduced by the prism or grating, may often be omitted in instruments of small power.
9) as we pass across the grating has been investigated by M.
Wards, as com pared with the phase they would possess were the central value of the grating interval maintained throughout.
If w now relate to the edge of the grating, on which there are altogether n lines, no- = 2a sin w, and the value of the last term in (I o) becomes no- sin 3w sin O'tan 0', - 1 1 - 6 mnX sin' w tan 0'.
In the Thies process, used in many districts in the United States, the vats are rotating barrels made, in the later forms, of iron lined with lead, and provided with a filter formed of a finely perforated leaden grating running from one end of the barrel to the other, and rigidly held in place by wooden frames.
Then suddenly the grating sound of a harsh voice was heard from the other side of the door, and the officer--with pale face and trembling lips--came out and passed through the waiting room, clutching his head.
The railings in fact do for sound what a diffraction grating does for light.
If, further, on leaving the grating the light be received by a focusing lens, e.g.
If we now suppose half the grating cut away, so as to leave 1000 lines in half an inch, the dispersion will not be altered, while the brightness and resolving power are halved.
A rectangular trough of boards, whose dimensions depend chiefly on the size of the planks available, is set up on the higher part of the ground at one side of the claim to be worked, upon trestles or piers of rough stone-work, at such an inclination that the stream may carry off all but the largest stones, which are kept back by a grating of boards about 2 in.