Continuing Part 1. At the beginning of the fourth movement of Pyotr Ilyich Tchaikovsky (1840-1893) Symphony No.6 – Pathétique (1893), the ensemble of the two violins is an interesting illusion.
The original scores of the first violin and the second violin are shown in figure (a) above. The notes are a bit jumpy and seem difficult to play. When the violins play together, we will hear the melody as shown in figure (b). In fact, there is no instrument that really plays the melody we perceive. The brain automatically reassembles the notes with closer pitch and more coherence into two new melody lines.
Diana Deutsch's book "Musical Illusions and Phantom Words" has mentioned that Tchaikovsky had an argument with conductor Arthur Nikisch. Nikisch thought it was difficult to play the original scores, so he rewrote the scores as shown in Figure (b). 
There was another reason for Tchaikovsky's persistence. According to the orchestral configuration at that time, the first and second violins were sitting separately on the left and right sides of the orchestra, instead of sitting in the adjacent areas on the same side as they are nowadays. In other words, if Tchaikovsky’s original scores are performed in old orchestral configuration, we may feel the melody dangling in space, which enriches the sense of listening .
In order to reproduce this effect, I edited the scores with MIDI and used panning the channels to compare with the version without panning. In the version without panning, because the two violin parts are of the same timbre, the effect is equivalent to the scores in figure (b), which is what we would have perceived.
There are two versions of panning: (1) Full panning: The first violin is completely on the left channel, and the second violin is completely on the right channel. This is suitable for being played by stereo loudspeakers, which is similar to separating two violin parts on the left and right sides of the stage. (2) Partial Panning: The first violin is panned to the left but not completely and the second violin to the right not completely. The first violin is still with a low level heard on the right and the second violin vice versa. This version is suitable for playing with headphones, because one ear cannot hear the other side's channel at all. The version of partial panning can achieve the effect like loudspeaker playback or live performances.
Full panning （for loudspeakers）
Partial panning （for headphones）
Although the effect of the melody dangling in the space can be heard, I still don’t understand why Tchaikovsky insisted so much. Did he really know the illusion hidden in this melody? Why not make the performance much easier? When I asked composer Zoe Lin about these questions, she gave me an answer I didn’t expect: The original scores would make the violinists frown, and the music would therefore sound much bitter. The rewritten scores are so easy to perform that there would not be such a deep emotion.
Somebody may have a question here: why do we have this illusion only in Tchaikovsky's work, but not in other music? This is because when our brain classifies sounds, the classification of timbre has priority over the classification of pitch (or melody). In general music, although there are often overlapping pitches of different musical instruments, we usually perceive the melody by following a particular instrument. Grouping effect on melody only appears when all the notes are played by the same instrument (or timbre). For example, dual-guitar configuration often appears in rock bands. The two guitars are usually tuned into different tones, so we can clearly distinguish what they play respectively. There will not be confusions and illusions.
 Deutsch, Diana. Musical Illusions and Phantom Words (p. 35). Oxford University Press. (2019)
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