On the 15th of February, 1786, I discovered that one of my planetary nebula, had a spot in the center, which was more luminous than the rest, and with long attention, a very bright, round, well defined center became visible. I remained not a single moment in doubt, but that the bright center was connected with the rest of the apparent disk.

On Nebulous Stars, Properly So Called, Philosophical Transactions of the Royal Society of London, Volume 81, 1791