"In order further to satisfy Your Honours"
In this letter, Leeuwenhoek responded to Thomas Gale's letter of March 2/12, 1686. In it, Gale passed along the comments of the members of the Royal Society to Leeuwenhoek's Letter 47 of October 12, 1685.
Leeuwenhoek structured the first part of this letter around his response to four quotations from Thomas Gale's letter. The quotations were in Dutch, so someone must have translated Gale's English.
1. Leeuwenhoek further explained his observations from Letter 47 of October 12, 1685. The first was about cotton seeds.
In order further to satisfy Your Honours on this matter, I have thought fit to put some cotton seeds - which I have had with me for over a year, and which are so old that their greenish colour has already faded - in water for one night, after which I removed from them their tough rind, being their first, and then their soft membrane, being their second envelope, and separated the leaves a little from one another.
To supplement his description, he enclosed two samples, which still exist. See Brian Ford's Leeuwenhoek Legacy for a detailed discussion.
8 or 9 of these seeds, from which the young cotton tree takes its origin, I send you herewith; on these a sharp eye will recognize, even without any magnifying glass, not only the four distinct leaves, together with that part which will become the root and stem, but one will also be able to see the small spots on the leaves.
I have moreover, removed the hard rind from two of such seeds (without these having lain in the water); they, together with their second envelope (which I have left on them) form a perfectly oval shape. I have cut one of these into twenty-five to twenty-six, and the other into twenty-eight to twenty-nine round slices, which, too, I send you herewith.
2. Leeuwenhoek further explained his observations of skin, the pores of which Leeuwenhoek did not see until thirty years later, when he described them in Letter XLIII of September 17, 1717. Back in 1685, he was defending his erroneous view.
If anyone should doubt this, and be inclined to see it for himself with his own eyes, then let him wash his hands clean, and dry them with a clean linen towel, and, after a little while, press hard or touch with his fingers on a pewter plate scoured clean, or on a clean dry glass, without moving the fingers to and fro on the plate or glass. This will leave, on such a plate or glass, blurred or dimmed places.
And if we examine these parts on the glass or plate through the magnifying glass, we shall find all the blurred spots to be nothing else than an incredible number of irregular fat particles from the fingers. But we shall find few fat particles, or none at all, where the cavities of the skin (which are called sweat-holes) have been, whensoever we might put this to the test.
3. Gale must have been referring to the nature of the discussion at the Royal Society's meetings, not noted in Birch's History:
Of another of your inventions it is to be feared that it will pass, in the judgment of most people, as hardly believable; namely, the one by which you find the slime of the eel, and other fish, not to be an excrement, but an essential and necessary part of their bodies, as well as having structures of its own; it is about this that the Society stands amazed...’, etc.
The skepticism that Leeuwenhoek's observations encountered among the less educated citizens of Delft was similar to what his observations encountered in the highly educated members of the Royal Society. Leeuwenhoek stood firm: if he saw something, it existed, not matter what anyone else believed. For him, it wasn't a matter of belief.
All I will say to this is that I had completely satisfied myself in my examination. But in order to satisfy also Your Honours thereon, I undertake to make some further examinations, and even to make a drawing of some very few vessels, as they appear to me through the magnifying glass, and to send you the same.
4. Gale's letter indicated to Leeuwenhoek the significance of his observations and the Society's trust in him to pursue them further. Gale wrote:
Your speculations concerning the mixture of several chemical fluids in the blood are in the judgment of the Society, highly deserving of being pursued further, in the hope that, by this means, some light may be given us towards discovering the secret, hidden action of some medicine present in the body, whose effect, however surprisingly, is not known otherwise than by its outward symptoms. This, therefore, they recommend to you for your further investigation, with our cordial wishes for a good result; for there could hardly be anything more beneficent for Humanity than the development of the sovereign art of medicine into firm scientific knowledge’.
"... there could hardly be anything more beneficent for Humanity." How else could Leeuwenhoek respond but "Since I see that such observations are held in high esteem, I shall continue my examinations"?
Finally, he had a response to yet another researcher. He wrote:
Mr. Robert Hooke requests me, in view of the lack of eggs of the silk-worm this springtime, to examine the seed of frogs, and to note the manner in which nature proceeds in the reproduction of these animals; for it may be assumed (says that Gentleman), or reasonably asserted, that the generation of most, if not of all, egg-laying fishes proceeds in the same way.
Referring to Letter 38 of July 16, 1683 (AB 72) to Christopher Wren, Leeuwenhoek wrote:
As regards this seed (or, to put it differently, these eggs) I already tried to examine it some years ago. But I found the eggs so dark, or blackish, that I was unable to observe them to my satisfaction.
Ever willing to please Hooke, who had done so much for him, Leeuwenhoek wrote:
However, I will undertake to resume my examinations anew; and if I discover anything worth noting in them, to communicate it. Since a few days ago I have examined the eggs of a small fish, and to my great pleasure I discovered therein something very special. But in order to say something about it I shall pursue it in other, (almost) similar, fishes; and I have given instructions to have them sent to me when they are obtainable.