LETTER 88. CONCLUSIONS Ao tT| 
quoted—by bacteriologists, and is usually said to be the first 
memoir in which bacteria of any sort are mentioned. How 
far this 1s true, readers of foregoing quotations can judge for 
themselves. References to this important epistle, addressed 
to Francis Aston, Sec.R.S.," are almost invariably given 
incompletely or incorrectly ; and this is doubtless due in 
part—though not altogether—to the fact that many versions 
of it are extant. Moreover, many different interpretations of 
the organisms described have been advanced, and this has led 
to some truly astonishing conclusions. Yet Leeuwenhoek’s 
words are, as usual, plain and straightforward, while the 
interpretation of his observations appears to me obvious. To 
anybody familiar with the flora and fauna of the human 
mouth they surely present no difficulties. I shall therefore 
give his own words (as well as I can) first, and shall comment 
upon them afterwards—adding exact references but ignor- 
ing many manifestly absurd statements made by other 
commentators. 
The passages in question run as follows: ? 
* Concerning Francis Aston little is now known. (He is not mentioned 
in the Dict. Nat. Biogr.) He was elected F.R.S. in 1678, and became 
Secretary in 1681—a post which he suddenly threw up in 1685 (ef. Rec. 
Roy. Soc., and Weld, I, 302). Afterwards he received a gratuity from the 
Society (Weld, I, 305), and on his death bequeathed to them his estate at 
Mablethorpe in Lincolnshire, together with his books and instruments 
(Weld, I, 428). His portrait now hangs in the apartments of the Society 
at Burlington House (at the foot of the staircase). For the following 
additional information I am indebted to Mr H. W. Robinson, Assistant 
Librarian of the Royal Society: Aston was born about 1644, and died in 
June or July 1715. He went to Westminster School in 1656, and was 
King’s Scholar in 1660 (aged 16). He entered Trinity College, Cambridge, 
in 1661, and became Fellow in 1667. (B.A. Cant. 1664/5: M.A. 1668.) 
He travelled abroad for some years, and in his youth was an intimate friend 
of Isaac Newton. 
* From Letter 39. 17 September 1683. To F. Aston. MS.Roy.Soc. 
Published in Brieven, Vol. I (Dutch), and Opera Omnia, Vol. II (Latin). 
The Dutch version was first printed in 1684 (Ondervind. en Beschouww. 
pp. 1-19), the Latin in 1695 (Arc. Nat. Det. pp. 41-53). Both versions are 
wrongly dated the 72th of the month (‘‘ den 12 Septemb. 1683” and “ pridie 
Iduum Septembris 1683”) apparently through a misreading of the MS. 
Owing, it would seem, to some oversight, two different English abstracts 
appeared in the Plil. Trans. The first was published in Vol. XIV (1684), 
No. 159, p. 568 [b¢s—a misprint for 598]: the second in Vol. XVII (1693), 
No. 197, p. 646. [The first—published 20 May 1684—is the fuller and 
better.] Hoole (1798) has given a third—and as far as it goes the best— 
