examine only one line at a time. The method of observation requires us to stop at each turn of the screw, and note the visibility of the fringes at each stopping-place. During the comparatively long time which it takes to do this the character of the radiations themselves may change. Besides, we have the trouble of translating our visibility curves into distribution curves. Hence it is rather easy for errors to creep in.

On account of these limitations of the interferometer method, attention was directed to something which shouldFIG. 96 be more expeditious, and the most promising method of attack seemed to be to try to improve the ordinary diffraction grating. The grating, as briefly explained in one of the preceding lectures, consists of a series of bars very close together, which permit light to pass through the intervals between them. The first gratings ever made were of this nature, for they consisted of a series of wires wound around two screws, one above and one below. This first form of grating answered very well for the preliminary work, but is objectionable because the interval between the wires is necessarily rather large, i. e., the grating is rather coarse. If we allow light to pass through these intervals, each interval may be considered to act as a source of light.