The finger should glide along the string as previously explained, the division of the notes in this instance being accomplished by a rapid spiccato movement of the bow, producing a series of short detached strokes. In a passage of this character it is impossible for either player or listener to discern whether the exact chromatic scale is actually played; the only method to approach anywhere near a faithful interpretation of the passage, is by a correct division of the bow strokes in groups of fours, at the same time carefully regulating the speed of the gliding finger. It will be evident that the gliding must be more rapid when the lower positions of the instrument are reached than in the high positions; thus in commencing the above passage (Ex. 52), the notes being nearer together in the higher positions than in the lower, the hand should move correspondingly slower; the movement becoming more rapid as the passage proceeds.
A third method of producing a chromatic passage--this time, however, with the left hand, without any assistance from the bow for the division of the notes--seems to be a speciality of Servais. It is introduced both in slow portamento effects and rapid chromatic passages.
The bow should be drawn as for a long sustained note, the division of the notes being caused by a series of rapid jerks performed by the left hand. The finger should sustain the pressure on the string during the whole passage, the movement is similar to that explained as the method of producing the vibrato, with the addition of the forward or backward progression of the hand according to the requirements of the passage. The passage may be played with any finger, but it is advisable to use the fourth finger for a descending passage (see Ex. 53) and the first or second finger for an ascending chromatic run. It is also here impossible to attempt to play the real notes except in very slow passages; to execute a rapid chromatic as above, the player should merely grasp the time of the semiquavers, and regulate the distances for each movement previously as explained.
The same effects as above examples are also possible in octaves, they are produced in a similar manner.
A style of bowing which seems to be either very little understood, or very much neglected, is "sul ponticello" bowing. In string quartets, orchestral music, etc., a very fine effect is possible if all the players execute it in a proper manner. The bow should be drawn quite close to the bridge, with only medium pressure applied, the stroke should be performed more rapidly than ordinary bowing. This bowing executed by a number of strings gives a very weird effect, the only objection to its use being that if great skill is not used, the string, instead of vibrating as a whole, will vibrate in segments, thus giving out one or other of the natural harmonics. It is stated that Paganini used to play certain passages in harmonics after the above manner; harmonic tones may be produced on any stopped note, by the bow alone, as follows. The bow should be drawn very lightly across the strings near the bridge, the left hand fingers firmly stopping the notes; various harmonics may be produced with one fingering by slightly varying the position of the bow, moving it slightly nearer or away from the bridge. Except for special passages, which are really intended for this kind of ponticello harmonic playing, it is not advisable to introduce it; the slightest irregularity in the position of the bow on the string will alter the harmonic notes to a great extent.