Rationale

Why divide Leeuwenhoek's scientific career into periods?

Anyone approaching Leeuwenhoek's output is put off by the complexity. For fifty years, he wrote almost three hundred letters with scientific observations. They cover a wide range of topics, to say the least, and an individual letter often shifted laterally to related topics if not to a different topic entirely.

Leeuwenhoek made no attempt to organize these observations other than chronologically according to his own numbering system. The letters are dated, but on several days he sent more than one letter. While this situation is convenient for understanding Leeuwenhoek's growth, it is inconvenient for almost every other purpose.

The letters have only numbers as unique identifiers, but we have three sets: Leeuwenhoek's 165, Cole's ~200, and Alle de Brieven / Collected Letters' ~365.

Rather than displaying as an impossibly long undifferentiated list, the letters divide here into seven periods of about seven years each.

How does one period differ from the next?

The letters themselves and Leeuwenhoek's treatment of them determine the limits of each period. Each of the four parts of the Works (Werken) is its own period. While I say four parts, they were bound over the years in either four and five separate volumes. Nor did the Latin volumes always correspond with the Dutch. The four parts here correspond to Leeuwenhoek's indexing of them. He indexed the first ten (Dobell #1-9) separately from the next four (Vervolg through Vierde Vervolg). The next four (Vijfde through Send-Brieven) all had separate indexes.

Dobell (1932 p. 394) had a set that divided them this way:

Clearly, putting Vijfde and Sesde with part II, 84-107, 24 letters, more evenly distributes the number of letters. It also better parallels the Latin volumes.

As shown on the table below, the other three periods, when most of the letters were published in Philosophical Transactions, come before and after those four and between Works III and Works IV.

Note

Leeuwenhoek published the Dutch edition of the letters in the same period in which he wrote them. For example, Letters 53 to 83 were written between January 1, 1687 and April 30, 1694 and the four volumes collecting them -- Vervolg, Tweede, Derde, and Vierde -- were all published between the same years. These events delimit the periods on the table below.

However, the Latin translations were not published during the years the letters were written, nor do the Opera Omnia collect the same letters as the Works (Werken).

Rationale for the division into seven periods