For an in-plane mounting, using a toroidal concave grating, we have found a condition giving stigmatic images for all wavelengths by means of the rotation of the grating and a slight movement of the incident and exit slits with increasing rotation angle, ε, of the grating. For this mounting, the central diffracted ray is at right angles to the central incident ray, and the distance,r′, between the exit slit and the centre of the grating is equal to the distance,r, between the incident slit and the grating centre. If the radius of curvature, R2, of the grating in the sagittal direction is chosen to be one-half of the radius of curvature, R1, in the tangential direction, and the relation r=(R1√2)sec & is maintained, a non-astigmatic image can be obtained regardless of the value of ε.