Work continues with emery 1m whose grains are comparable to those carbo 100 or 120 , but that leaves less deep fractures because of its lower hardness . Now we will adopt general way for the rest of the work, a normal stroke on which it must attract the attention specially: they consist a back and forth roughly centers (Fig 22 and 23), the total amplitude is about 1/3 of the disk diameters (therefore it is necessary to exceed one sixth of each side , 3 to 4 centimeters with a mirror of 20 centimeters), with a lateral departure constantly variable limit 1/8 at the each side. the shape of the strokes may well affect that of a V or W or of a loop more complicated ∞ as an example ( G. W. Ritchey ) . Every five or six strokes it turns a little the upper disk in his hands and one turns oneself as in roughing(coarse grinding) . What is essential in all this is only to respect roughly 1/3 the average amplitude and to vary as most p...