A discussion of the elementary theory of the astigmatism of the concave grating. A spherocylindrical lens placed in front of the plate serves to produce a stigmatic image of the slit and increases the speed of the instrument by about 25× and doubles the dispersion, allowing the full theoretical resolving power to be realized.
J. B. GREEN and R. A. LORING, "An Auxiliary Lens for Use with the Concave Grating," J. Opt. Soc. Am. 24, 348-348 (1934)