The first requisite for this is a straightedge of iron or brass, as true as possible. This ruler must be laid on the stone in various directions, and the lithographer must note all parts where there is space between the straightedge and the stone. The greater the space, the greater the unevenness of the stone; and those that show especial unevenness should be set aside from those that have little. When this has been done, the very uneven stones must be ground with a coarse sandstone and plenty of water applied to the elevated places till the straightedge can be applied in all directions without showing any material interstices. Then these ground stones may be placed with the others that were fairly even in the beginning. Now we take one of these stones, and lay it on a strong, firm table, the best being one to be described later. Finely grained sand is sprinkled over its surface. In the absence of sand, a substitute can be made by powdering a common sandstone of the kind used for coarse grindstones. A spoonful of water is poured over this. A little soap may be mixed with the sand. It facilitates the grinding and makes the sand take hold of the stone better. Now another stone is laid on the first one, and is moved back and forth continually in all directions. The sand and water must be renewed often. Thus both stones, the upper and lower, will be ground simultaneously, and very evenly and true, if the work is done right. One must take care never to draw the upper stone far beyond the lower one, because that would throw the centre of gravity of the upper plate too near its ends, as a result of which the upper plate would become concave and the lower plate convex. To avoid this defect, the upper plate should be moved around only in small circles. It is good also to change the plates around frequently, so that the upper shall be the lower. Another good plan is not to use two stones of equal size, but to take for the upper stone one only half as large as the lower. It is necessary also that the straightedge be applied frequently. The stone must always be cleansed thoroughly before this test. Once one has the proper experience, it is possible to tell by mere touch if the plates have been sufficiently ground. So long as they still have uneven spots, a certain resistance is noticeable, so strongly sometimes that it is impossible to move the upper plate further without lifting it and sprinkling new sand. Sometimes this friction is so great that manual strength does not suffice to separate the stones, especially if they happen to dry. If tools are used to separate them, it happens often that pieces are torn from the stones, because they adhere so mightily. In this case a very simple and convenient remedy is the best. An ordinary table-knife is inserted gently and then tapped very lightly, when the stones will separate at once.