In 2017 I worked out how to play 19-equal divisions of the octave on a standard french horn which is pitched in F and B-flat. My solution was to flatten the 1st note of the scale using the right hand. I started on written F (top of the treble staff), which is concert B-flat. Bending this pitch by 63 cents and all lower octave written Fs is easy, though there is a change of tone colour.

The 2nd valve slide is lengthened enough to produce the longer semitone, below written F, of 126 cents, which is not too far from its normal position for tuning a semitone at 100 cents.

The 1st valve slide is shortened slightly to produce a tone lower than written F, of 189 cents, which is 11 cents higher than the standard 200 cent whole tone.

I then tuned the next lower note by using the right hand. 

The whole scale of 19 notes can be done with 7 right hand adjustments or less with alternate fingerings (tube length choices).

Recently I purchased a second hand horn in B-flat with an extra valve and slide. The valve is operated with the thumb and my colleague Ed Diefes, who plays tuba and does brass repairs, shortened the slide so that it can provide good quartertones (50 cent intervals) and also good intervals of 63 cents for playing in 19-division tuning.

This means the tone quality of each note in the octave is a standard quality and other tonal qualities can be ascribed for reasons other than simply playing the required pitch.