A number of tests of a large diffraction grating are described. It is shown that useful information about the performance of the grating in a spectrometer can be obtained from an analysis of the diffracted wavefront by the method of Stroke. Comparison of the instrumental profile at three wavelengths strongly suggests that the influence of changes in the angle of diffraction on the profile cannot be neglected. A resolving power of 6.9 × 105 is attained at λ 6328 A. At the same wavelength the first order Rowland ghosts in the ninth order of diffraction have an intensity of only 8 × 10-4 of the parent line. The influence of stray light on the central intensities of deep Fraunhofer lines is shown to be small but by no means negligible.