CHRONOLOGICAL NOTE 
The Gregorian correction of the Julian Calendar was not made simul- 
taneously in England and the Netherlands, and during Leeuwenhoek’s 
lifetime two different systems of dating—known as ° Old Style ” and “ New 
Style ’’—were current in these countries. 
“New Style” dating was adopted in the Provinces of Holland (wherein 
Delft is situated) and Brabant on 5 December 1582, which was called the 
15th of the month—thus advancing the Calendar by 10 days: though other 
Provinces of the Netherlands (Gelderland, Utrecht, Overyssel, Groningen, 
Friesland) did not recognize the new system until 1700. But in England 
the change was made even later, for it was not until 1752 that the Old 
Style ” was abolished by Act of Parliament. The discrepancy between the 
Julian and the Gregorian systems having become 11 days in the year 1700, 
the English Calendar was corrected by calling the 3rd of September (1752) 
the 14th. At the same time, the old Anglican method of commencing the 
year on March 25 (instead of January 1) was ended. Good Friday was 
New Year's Day in Delft before 1575, but not afterwards. 
All Leeuwenhoek’s own dates are therefore “New Style’’, and agree 
with modern reckoning. English dates of his period are, however, usually 
“Old Style’, and consequently to be understood as 10 days (from 1632 to 
1700) or 11 days (from 1700 to 1723) later in actual time. In the present 
work all dates of importance are given, as far as possible, in present-day 
form (N.S.) unless old-style dating (O.S.) is indicated. 
(vi) 
