individual apertures of the mask therefore etchant which pentrates the holes from the inner side of mask 10 tends to flow back through the holes and contribute to the desired etching it is frequently desirable to grade the holes from the largest size at the center of the mask to a decreasing size with radial distance from the center this is easily accomplished by arranging spray heads in the manner indicated by the dots in fig3 arranging them in concentric circles spaced to achieve uniform coverage of the field of apertures grading control is easily exercised by changing the separation of the spray heads from the portion of the mask to which they are individually to direct etchant and it is also subject to control by varying the flow of etchant through the various spray heads as a practical matter it is convenient to feed the spray heads from a common manifold and exercise control by spacing the spray heads with respect to the plane of the mask alternatively a stick or a single line of spray heads 20 may be employed in the manner indicated in fig2 such an arrangement provides a ribbon pattern of etchant having a width that is small relative to the smallest dimension of the field of apertures and having a length which is at least equal to the largest dimension of that field the linear array of spray heads in this embodiment is rotatably mounted and is continuously driven to rotate the pattern of etchant about an axis that is normal to the central area of mask 10 rotation of the nozzle array causes the entire field of apertures to be sprayed as necessary to etch back or enlarge all of the apertures of the field a suitable speed of rotation is 30.