We find that a third line RS also lies on a series of crests, and therefore a plane wave sets out in the direction perpendicular to RS. We notice here that the crest from A is two wave-lengths behind that from B, and so on, and therefore if [Greek: beta] is the angle between RS and PQ, CD sin [Greek: beta] is equal to two wave-lengths. Similarly we get another plane wave for a three wave-lengths difference, and so on. The intensity of the wavelets falls off fairly rapidly as they become more oblique to their original direction, and therefore the intensity of these plane waves also falls off rather rapidly as they become more oblique to the direction in which PQ goes. We see that the essential condition for the plane wave to set out in any direction, is that the difference in the distances of the plane wave from two successive slits shall be exactly a whole number of wave-lengths. Should it depart ever so little from this condition we should see, on drawing the line, that there lie on the line an equal number of crests and troughs, and therefore, if a lens focus waves in this direction, the resulting effect is zero. The directions of the waves PQ, LM, RS, &c., will therefore be very sharply defined and will admit of very accurate determination. +Dispersion by Grating.+--Evidently the deviations [Greek: alpha], [Greek: beta] will be greater the greater is DE, _i.e._ the greater the wave-length, and therefore the light or heat will be "dispersed" into its different wave-lengths as in the prism; but in this case the dispersion {76} is opposite to that in the normal prism, the long waves being dispersed most and the short waves least. Evidently, too, the smaller the distance CD the greater the angle, and therefore for the extremely short wave-lengths of light and of ultraviolet rays we require the distance between successive slits to be extremely small. [Illustration: FIG. 26.] +The Spectrometer.+--The grating is usually used with a spectrometer, as shown in plan diagrammatically in Fig. 26. The slit S from which the waves radiate is placed at the principal focus of the lens L, and therefore the waves emerge from L as plane waves which come up to the grating G. The telescope T is first turned until it views the slit directly, _i.e._ until the plane waves like PQ in Fig. 25 are brought to a focus at the principal focus F of the objective of the telescope. The eyepiece E views the image of the slit S which is formed at F. The telescope is then turned through an angle, [Greek: alpha], until it views the second image of the slit which will be formed by the plane waves similar to LM in Fig. 25. The angle [Greek: alpha] is carefully measured by the graduated circle on the spectrometer, {77} and hence the wave-length of a particular kind of light, or of a particular part of the spectrum, is measured. This spectrometer method is exactly the method used for measuring the wave-lengths in the visible part of the spectrum.