LEEUWENHOEK’S MICROMETRY 335 
diameter, and therefore some 870 ». These magnitudes seem 
to agree well with all Leeuwenhoek’s inferences—so far as 
I have been able to check them. 
A Red Blood-corpuscle (of man) measures about 7°5 u 
in diameter, and Leeuwenhoek’s frequent choice of this 
structure as a standard of size has been amply confirmed by 
all later microscopists. Even today we commonly see an 
outline of a human erythrocyte inserted among drawings 
of microscopic organisms as an indication of their relative 
magnitude. 
It may be remarked here that Leeuwenhoek himself had a 
very good idea of the actual diameter of a red corpuscle, though 
he could not express it exactly in terms of any micrometric 
unit : for he notes in one place’ that he had satisfied himself 
that 100 diameters of a red corpuscle amounted to something 
less than that of a coarse grain of sand (which he had just 
assessed at 35 inch). Consequently, he imagined the diameter 
of a corpuscle to be rather less than 3500 of an inch—an~ 
astonishingly good estimate.” 
The Vinegar-Hel (the nematode Anguillula aceti) is 
assigned various sizes in the text-books. I have cultivated 
and studied this worm at various times, and find that ordinary 
large individuals (females) may measure anything from about 
1200 to 1700 in length. “A full-grown eel such as we 
see in vinegar’”’ is approximately 1°51um. long, and this agrees 
quite well with all Leeuwenhoek’s references. 
A Millet Seed is more difficult to appraise. There are 
now many kinds of millet (Panicum miliaceum)—*a name 
applied with little definiteness to a considerable number of 
often very variable species of cereals, belonging to distinct 
genera and even subfamilies of Gramineae”’.* I have measured 
1 Letter 42, p. 32 of Dutch edition. 
* This was pointed out by Harting in 1850 (Het Mikroskoop, III, 404), 
and again by myself (1920)—in ignorance of his earlier annotation. 
Haaxman (1875, p.56) makes the absurd mistake of commending L. for 
estimating the diameter at so (instead of goo) of an inch. 
3 Encyclopaedia Britannica, 11th ed. (1911). It is rather surprising that 
“a, millet seed” is still so frequently referred to, as a standard of size, in 
biological writings (especially text-books). I have asked many people how 
big a millet seed is, but have never yet found anybody who could tell me 
even approximately—including one distinguished person who had himself 
used the expression as a descriptive term. 
