Instead of using the refraction of light in a prism to separate the wave-lengths, we can use the interference which arises when a bundle of parallel light waves passes through a ruled _grating_, consisting of a great many very fine parallel lines, equidistant from each other; such a grating can be made by ruling lines with a diamond point on the metal coating of a silvered plate of glass. From each line there are sent out light waves in all directions; but if we are considering light of one definite colour (a given wave-length, _monochromatic light_), the interference among the waves from all the slits practically destroys all waves except in the direction of the original rays and in the directions making certain angles with the former, dependent upon the wave-length and the distance between two successive lines (the grating space). Monochromatic light can be obtained by using as the source of light a spirit flame, coloured yellow with common salt (sodium chloride). If the slit in a spectroscope is lighted with a yellow light from such a flame, and if a grating normal to the direction of the rays is substituted for the prism, then in the telescope there is seen a yellow image of the slit, and on each side of it one, two, three or more yellow images. If sunlight is used the central image is white, since all the colours are here assembled. The other images become spectra because the different colours are unequally refracted. In these _grating spectra_, which according to their distance from the central line are called spectra of the first, second or third order, the violet part lies nearest to the central line, the red part farthest away. Since the deflection is the greater the greater the wave-length, then violet light must have the shortest wave-length and red the greatest. From the amount of the refraction and the size of the grating space the wave-length of the light under investigation can be calculated. For the yellow light from our spirit flame the wave-length is about 0·000589 mm. or 0·589 μ or 589 μμ. In centimetres the wave-length is 0·0000589 cm.; from the formula ν = _c_/λ, ν = 526 × 10¹². The frequency is thus almost inconceivably large. For the most distant red and violet in the spectrum the wave-lengths are respectively about 800 μμ and 400 μμ, and the frequencies 375 × 10¹² and 750 × 10¹² oscillations per second.