After roughly shaping the head comes the delicate operation of "setting." This is also known as putting in the "spring" or _cambre_. The principle upon which the amount of curve is determined is that an imaginary straight line drawn from the face of the head to the face of the nut shall coincide with the stick at the point of its greatest deviation from the horizontal. There is no fixed distance from either end for this extreme point of deviation to occur. It is a matter that rests entirely on the judgment of the maker, who, if thoroughly experienced, regulates the curve by any variation in rigidity he may discover in the stick. Thus should his observations point to the fact that a certain portion of the stick is slightly weaker than the rest, there will he put the greatest amount of "spring." It must be understood, however, that a good maker never uses a stick that is palpably unequal. He will only take this trouble to correct infinitesimal weaknesses (discernible only to a hand of great experience) in wood of exceptionally good grain. It is astonishing how many violinists seem to think good bows are made by accident. Few know that there are some men who can _make_ a fine bow. The prime factor in the "setting" of a bow is heat, by the judicious application of which the straight rod is made to assume and retain the desired _cambre_. The heat used now-a-days is that produced by an ordinary gas flame. Dry heat is absolutely essential, as the slightest moisture draws all the pigmentary matter out of the cells in the wood and leaves the bow as colourless and mean in appearance as a stick of deal. As it is, with dry heat even, the amount of colour exuded by a good stick during this process is quite enough to stain the hands a deep purple. The great point to be observed in "setting" a bow is to make sure that the fibres are all heated equally right through to the centre of the stick. If this does not receive sufficient attention the bow can not possibly retain its curve, for the inner fibres that have not been affected by the heat will always be trying to resume their original straight position, and are bound ultimately to overcome the resistance of the heated outer fibres, with the result that the bow either becomes straight or warped and twisted, most probably the latter. To understand that this must be so it is only necessary to remember that any elastic rod, a walking stick for example, can be held so as to form a curve but as soon as the pressure is released it immediately recovers its normal state. This is what happens with the unheated inner fibres in an inferior bow. The constant strife of opposing forces _must_ result in victory for the active force of the inner fibres over the passive resistance of the heated outer fibres.