RULING TRANSMISSION GRATINGS.
The grating just considered is termed a transmission grating. The arrangement of alternate transparent and opaque strips is obtained most easily by ruling fine lines with a good diamond on prepared glass surfaces. The transmitting portion is the unscratched part, the lines made by the diamond diffusing the light and acting as the opaque strips. This ruling is a very troublesome and delicate operation. First tance from one aperture to the next. This is equal to the sum of the width of the aperture and of the material separat ing the apertures. if this distance be called d (see Fig. 772), i3 represents the retardation or difference of phase between two similarly placed rays. Triangle Ka b zm is similar to a z m ---= sin 8 where 0 is the angular deviation of any ray. Nov for a maximum of light is equal to 2n i.e. an even number of half wave lengths where a is any number and x de notes wave-length. The equation may therefore be written X = sin 0 for a bright line. Here, then, is a simple re lation between the wave-length and the deviation. The distance d can be made any size, or rather, since it is so small, the diamond point has to be carefully selected. Its scratch is carefully ex amined under the microscope. It may not give a steady cut, and perhaps only cuts one way. If faulty in any particular
it has to be rejected. The best points, too, are often spoilt after ruling only a few lines on glass, owing to its compara tive hardness. The spacing of the lines is automatically adjusted somewhat after the manner of the feeding of the cutter in a planing machine. The glass which is being ruled must also be kept at a steady temperature, as any expansion or contrac tion necessarily alters the spacing of the lines, though this effect is of much less importance than in the case of metallic gratings, described later on. Rutherford was one of the first investigators who produced reliable gratings. The distance d is obtained by counting carefully under a microscope the number of lines in a given distance and dividing the latter by the former. It will be seen from the formula given that the smaller the dis tance d is made, the greater the angle H. Therefore everything possible is done to obtain the largest number of distinct, even lines in the smallest space. The ordinary grating used with a. spectrometer in laboratory work has no less than two or three thousand lines per inch. It has, however, been found comparatively easy to rule up to 14,000 lines per inch.