334 LEEUWENHOEK AND HIS ‘“‘ LITTLE ANIMALS” 
the inches subdivided into tenths, was his standard. Fortu- 
nately, he had this engraved for one of his later letters,’ so 
that it can now be measured. From the picture it appears 
that his “ inch’”’ was approximately 26°15 mm., and therefore 
slightly greater than the modern English inch (25'4 mm.).” 
His own drawing of a cubic inch (p. 189 supra) confirms 
this determination. 
The commonest objects with which Leeuwenhoek com- 
pares ‘“‘animalcules” are a sandgrain, a human red_blood- 
corpuscle, a vinegar-eel, and a millet seed. He also used 
other more or less verifiable measures, however, such as “a 
hairsbreadth,” “‘the diameter of a louse’s eye,” “the bigness 
of a hair on a louse,” etc. J may say a word on each of these. 
Sandgrains are Leeuwenhoek’s common standard of com- 
parison. At first sight it seems impossible to translate these 
highly variable structures * into exact modern measurements, 
but it is actually—from the information which he supplies— 
quite feasible. He generally referred to two kinds of sand— 
coarse and fine. A fine sandgrain, according to the letter 
translated on p. 188, was about #5 inch in diameter : but else- 
where‘ he states that 100 of his very small sandgrains, laid 
end to end, equalled about an inch. This seems to be the 
usual magnitude he had in mind—7.e., about 745 of an inch, 
or (using his scale) approximately 2604. A coarse sandgrain, 
according to his own statement,’ was about 35 inch in 
1 Send-brief XXVIII, to Boerhaave. 28 September 1716. Also published 
in Latinin Epist. Physiol. and Op. Omn. The plate faces p. 271 of the Dutch 
edition, and p. 266 of the Latin. 
2 The scale in the figure is 5 inches long, and is accurately delineated. 
A fair average can therefore be readily obtained for 1 inch. But as the 
actual engraving was made on copper, and the prints are on paper (which 
was wetted, and then shrank somewhat), I have made my estimate from 
4 different prints in my possession (which differ only very slightly from one 
another). The mean for 1 inch (here given) is therefore derived from the 
combined measurements of 20 printed inches. 
3 Ty. said himself (Send-brief XLI, 26 Aug. 1717) that “as big as a grain 
of sand” is an inaccurate expression, andit would be better to use a millet 
seed or a mustard seed for comparison. 
4 Letter 35, to R. Hooke. 3 March 1682. Published in Dutch and 
Latin works. 
> Letter 42, to the Royal Society. 25 July 1684. Published in Dutch 
and Latin works. 
