A failure of the second and third orders from a concave grating to focus in the same curve is described. The fourth and fifth orders show no anomaly. No logical explanation seems possible at present but it is pointed out that the effect may be common though undetected.
A. G. SHENSTONE, "New Vagary of a Concave Grating," J. Opt. Soc. Am. 53, 1253-1253 (1963)