Wrote Letter L-021 of 1675-01-22 to Henry Oldenburg about a new technique for observing blood and sectioned brain tissue; optic nerve, smoke, gunpowder, and scorpions

January 22, 1675
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Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

Leeuwenhoek wrote this letter to Henry Oldenburg about a new technique for distinguishing blood-corpuscles; a new method for examining brain tissue. Optic nerve was re-examined lengthwise. Tobacco and tobacco-smoke. Experiments with saltpetre and gunpowder. The sting of scorpions and their poison.

In his own defense, Leeuwenhoek addressed the doubts expressed in a (lost) letter from Oldenburg on the previous Christmas Eve, 1673-12-24:

You have the goodness to recommend me to take care not to be deceived by my method of observation. Sir, be assured that my manner of observation has not, for aught I know, misled me and that the little glass tubes formerly used in my observations were only made immediately before use when I wanted to make a careful observation, and they were not allowed to be touched by hand or finger on the spot I wanted to study, and now those tubes are rarely or never used by me. At present I use quite a different method of observation, which is as reliable as can be invented (barring improvements) and if I should be inclined to make this method known, I do not doubt that you and all the Gentlemen Amateurs would esteem my instruments and method of observation.

In this letter, Leeuwenhoek also discussed other techniques, most notably confirming that he was among the first to use sections:

In order to satisfy you and the Gentlemen Amateurs I have again observed the brain of a cow etc. not in glass tubes as previously, but I cut off several pieces with a sharp razorblade, and studied these as such. ...

I take the liberty to send you three or four transverse sections of nerves.

These are the sections that Brian Ford found and discussed in The Leeuwenhoek Legacy.

Leeuwenhoek described an experiment that he began by putting a grain of gunpowder in a closed glass tube. When he ignited it, noting happened. He added a second and third, and again, nothing happened.

For the fourth time I enclosed a large grain of saltpetre in a much smaller glass, and igniting this, the glass burst, giving so loud a report as if a pistol were fired, and the pieces burst in my face and endangered my eyes, but I was so - lucky as to suffer only some pain in my eyes and a red spot on each cornea that disappeared next day. This is a warning to anybody who should want to repeat these observations.

This letter also has the famous black taffeta metaphor. According to Dobell (p. 331), this statement implies that Leeuwenhoek used what we now call dark-ground illumination to more clearly observe minute objects:

I can observe the globules in the blood as clearly as if we saw with our eyes without the help of glasses the grains of sand on a piece of black taffeta.

Leeuwenhoek did not explain how he obtained the dark ground, perhaps because it was so easy. If he viewed a glass tube full of transparent liquid by holding it up directly to the window, he had a bright background. If he pivoted slightly in either direction away from the window in the direction of one of the two walls, he had a dark background. It might have been obvious to Leeuwenhoek, but it was not as easy for the people using the double-lens microscopes at the time to make a dark background for the specimen.