It was Albert Abraham Michelson (1852-1931) who imagined that success in that direction could be obtained by creating an arrangement with which, in the direction of maximum intensity of the light diffracted by the single segments, the difference in path between the two consecutive beams was different from zero and, so that the order of the spectrum and thus the resolving power would be high, that difference had to be quite a large one.
He arrived at these results in 1898 with the echelette grating which bears his name and is composed of a stack of homogeneous, perfectly flat and parallel plates of glass having the same thickness and width but with decreasing length arranged in steps in such a way that all the steps have the same height, width and depth.
The plates of glass are held tightly together so as to represent, from the optical viewpoint, a homogeneous block.
The height of the steps, which coincide with the thickness of the plates, is on the order of one centimetre and their depth, which is given by the difference in length of adjacent plates, is on the order of a millimetre.
The echelette grating, like any other dispersive apparatus, is placed on the platform of a spectroscope, between the collimator and the telescope, so that a parallel beam of light to be analysed is normally incident on the longest plate.
The beam goes through the block of glass and is diffracted by the single steps which act as a series of parallel slits having the same width, shifted with respect to each other by the same distance in the direction perrpendicular to the plane to which it belongs, as occurs on the flat grating in which they are shifted along the plane to which they belong. ...continues