‘By a well-known photographic process a transparent tracing of the undulations of the spiral can be represented by a line of similar dimensions on some resisting substance, tempered steel for example. ‘When this is done, this resisting surface is placed in a turning machine which causes it to revolve and advance with a velocity and motion similar to those by which the registering surface was actuated. A metallic point if the tracing is concave, or a grooved index if it is in relief, is kept upon the tracing by a spring, and the index which supports this point is connected with the centre of the membrane which produces the sounds. Under these conditions, the membrane will be actuated not by the vibrating air, but by the tracing which guides the index, and the impulses will be precisely similar in duration and intensity to those to which the registering membrane was subjected. ‘The spiral tracing represents equal successions of time by increasing or decreasing lengths. There is no inconvenience in this, since the turns of the spiral are very close together, if only the circumference of the turning circle is used; but then the central surface is lost. ‘In all cases the tracing of the helix on a cylinder is much more satisfactory, and I am now trying to make this idea practicable.’ [20] Never make a contact between the stylus and the cylinder until the latter is covered with the tinfoil. Do not begin to turn the cylinder until assured that everything is in its place. Take care, when the stylus returns to the point of departure, to bring the mouthpiece forward. Always leave a margin of from five to ten millimètres on the left and at the beginning of the sheet of tinfoil; for if the stylus describes the curve on the extreme edge of the cylinder, it may tear the sheet or come out of the groove. Be careful not to detach the spring of the caoutchouc pad. To fix the tinfoil, apply varnish to the end with a paint-brush; take this end between the finger and thumb of the left hand, with the sticky part towards the cylinder; raise it with the right hand and apply it quite smoothly to the cylinder; bring round the sticky end, and join them firmly. To adjust the stylus and place it in the centre of the groove, bring the cylinder to the right, so as to place the stylus opposite the left extremity of the tinfoil; bring forward the cylinder gently and by degrees, until the stylus touches the tinfoil with force enough to imprint a mark. Observe if this mark is quite in the centre of the groove (in order to do this, make a mark with the nail across the cylinder), and if it is not, adjust the stylus to the right or left by means of the little screw placed above the mouthpiece. The depth of the impression made by the stylus should be ⅓ millimètre, just enough for it to leave a slight tracing, whatever the range of vibrations may be.