Abstract
Mack, Stehn and Edlen have treated the aberrations of a concave grating at large angles of incidence for the case in which both slit and plate are on the Rowland circle. The present paper shows that, by allowing minor departures from this position, it is possible to reduce the aberration to one-fourth of the value found by them. This increases the optimum grating size and maximum resolving power by a factor of 2½. Certain conditions are discussed under which an even greater increase in- the size of the grating is desirable.

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