Looking at the detail of C86.10 we can see the letters have much more clearly defined thick and thin strokes, the result of using a broad edged pen. In C86.9 Johnston is either using a brush or a very badly cut quill pen to write with, the result is blobby whereas the effect in C86.10 is one of greater crispness. Johnston would come to define 'Sharpness', (that is explicit contrast between thick and thin strokes), as one of the essential qualities in good writing. This is the beginning of his search for this quality.
The achievement of sharp writing depends partly upon the crisp and true edge of the pen you use and partly on the combination of writing surface and ink. Johnston's notes of this time contain diagrams of how to cut a really sharp goose quill (see H. Child, Ed. The Calligraphers Handbook for contemporary procedures). Later Johnston would experiment with mixing different kinds of ink to get the sharpness he required, today we mostly use Japanese stick ink to achieve a sharp result.