There is clearly no theoretical limit to the resolving power of gratings, even in spectra of given order.
For accurate measurement of the displacements of lines of stellar spectra which are produced by the relative motion of star and observer in the line of sight, a very beautiful instrument has been devised by Dr J.
Complex spectra of stars of the solar type this is by no means the case; for, as Dr Hartmann remarks, " in the first place the lines in these spectra are so numerous that their complete measurement and reduction would require many days, and in the second place a rigorous reduction of such material has hitherto not been at all possible because the wave-lengths of the lines are not known with sufficient accuracy.
Attacked in detail, they vanish one after another into as many teasing spectra of uncertainty.
He constructed a map of as many as 576 of these lines, the principal of which he denoted by the letters of the alphabet from A to G; and by ascertaining their refractive indices he determined that their relative positions are constant, whether in spectra produced by the direct rays of the sun, or by the reflected light of the moon and planets.