My dear Theo,
I just received your letter and the enclosed, for which many thanks. Wanted to reply to you straightaway that I repeatedly came across sayings by Diderot, and also think that he’s perfectly in tune with his age.
With him it’s as it is with Voltaire himself; if one reads a letter from these fellows, preferably about the most prosaic things or about nothing, there’s an alertness and a sparkling spirit in them that charm. Let’s not forget that they made the Revolution, and it took genius to carry their age along with them and to get minds that are epicene and passive to strive in one direction towards a goal. So I have every respect for them.1
You’ll shortly receive two studies of the autumn leaves, one in yellow (poplars) — and the other in orange (oaks).2 I’m utterly preoccupied with the laws of colour. If only we’d been taught them in our youth!
But most people’s story, through a sort of fate, is to have to seek the light for a long time. For it’s absolutely certain that the laws of colour, which Delacroix first ordered and put forward in full and in context to the general benefit — as Newton did gravity and as Stephenson did steam — that these laws of colour are a light.3  1v:2
I’ve also made another autumn study of the pond in the garden at home.4 There’s definitely a painting in that spot. I did already try to get it out once last year.5
The one I’ve made now is something of a stiff composition; two trees (orange and yellow) on the right, two bushes (grey-green) in the middle, two trees (brownish yellow) on the left. In front — the pond, black — foreground of withered grass. Background, a glimpse over the hedge onto a very bright green. A sky to harmonize with this in terms of power, in slate-grey and dark blue. They’ll6 certainly think it too black and too dark, but the time when one makes dark studies is always too short.
I’m enclosing the book by Charles Blanc7 in the crate with the studies — also a Bible they gave me for you at home, of which I made a still life.8
Don’t let it bother you if I just leave the brushstrokes on my paintings as they are, with smaller or larger protrusions of paint. This doesn’t mean a thing — if  1v:3 one leaves them a year or so (or half a year is enough) and scrapes over them quickly with a razor blade, one gets much more permanency of colour than would be the case if the paint were put on lightly. If a painting is to remain good and keep its colours, it’s important that the light areas, in particular, are painted on heavily. And both the old masters and the French painters of today have done this scraping off. I believe that glazes of a transparent colour often sink in altogether and disappear over time if they’re applied before the painting in its preparatory phase is thoroughly dry, but applied later, they really do endure. You yourself made the observation that the colour of my studies in the studio got better rather than worse over time. I think this comes from putting the paint on heavily, where I don’t use any oil. When it’s a year old, the little oil that the paint always contains has sweated out and then one gets the good, solid impasto. That’s a question — painting so that it hardens properly — that it rather comes down to, to my mind — it’s a pity that some enduring colours like cobalt are so expensive.  1r:4
I don’t know what to think about the chromates and madder, but I can well imagine that some, particularly American, sunsets — you know those sorts of paintings that are obtained with glazes of chromates — last a terribly short time.
Daubigny and Dupré, on the other hand, endure. Isn’t it curious that the Vermeer of Delft in The Hague has retained its colour so magnificently, with a whole series of strong tones of red, green, grey, brown, blue, black, yellow, white?9
The painting by Haverman in Amsterdam that you’ll remember — (as not good) is badly painted, fatally badly with a view to time, I fear.10 I just mention it because he, so they say, is awfully respected precisely for his technique. But — it’s painted, well I’d say like, for instance, Ary Scheffer or Delaroche painted in terms of technique — and — lovers of sound, good, powerful work have always had quite a lot to say about that. Now those paintings smoothed out with oil &c. are cracking horribly — I noticed it in the Fodor.11 Yet Silvestre says that Delacroix drenched his paintings with oil — ‘bathed them in oil12 — but — what I do imagine is that this was heavily impasted work, first worked up in full impasto, then left for a year — and then, if the paint had perhaps became rather too dull and rough — yes — then Delacroix will have saturated those paintings with oil later — but only after they were dry to the core. Then it can’t do any harm. Regards.

Yours truly,

Hasn’t there been a Lhermitte this month?13

I’m really looking forward to De Goncourt.14