336 LEEUWENHOEK AND HIS ‘‘ LITTLE ANIMALS ”’ 
samples of modern “ brown” and “ white ” millet (commonly 
sold for bird-seed) and found that the grains averaged 1°8 mm. 
and 2:17 mm. respectively: but as they are not spherical, 
their “diameter” is difficult to determine. In one place’ 
Leeuwenhoek estimates a green pea to have a diameter equal 
to 4} millet seeds, while elsewhere” he says that a pea weighs 
8 grains. On selecting a few fresh green peas of about this 
weight (520 mg.) and measuring them, I found their diameters 
‘to be rather less than lem. (But here again—as peas are 
not round—it is impossible to express their diameter, or 
“axis”? as he called it, with precision.) If Leeuwenhoek’s 
millet had a diameter of about # of such peas, it must have 
measured roughly 2mm. across. This agrees with my 
estimates from modern millet, and does not seem to disagree 
with any of his own statements. I assume, therefore, that 
when he uses “a millet seed” as his standard, he means a 
spherical body approximately 2 nm. in diameter. 
A Hairsbreadth is estimated by Leeuwenhoek himself? as 
equal to g3o of an inch (about 43°6 w on his scale). The hairs 
measured were plucked from his wig—not his own head—and 
human hairs are usually much coarser.” 
The Eye of a Louse’ appears to be rather an indefinite 
standard of size. I find, however, that it can be estimated 
approximately. The eyes of Pediculus humanus corporis (the 
louse which Leeuwenhoek particularly studied) have fairly 
uniform dimensions in both sexes. They are not spherical; 
but series of measurements made longitudinally and trans- 
versely give closely similar mean diameters. The average for 
‘an eye of a big louse’’ I have found to be about 70 u—ranging 
from 64 » to 80 w. It can thus be said, with a fair approxima- 
tion to the truth, that ‘‘a louse’s eye’ has a diameter 10 times 
that of a human red blood-corpuscle. This is in good agree- 
ment with Leeuwenhoek’s interpretations. 
1 See p. 169 supra. 
> See p. 214 supra. 
3 See p. 189 supra. 
+ Most of my own hairs, which are unusually fine, have diameters at 
least twice as great. I have never seen a human hair (from the head) 
measuring only 43 u in diameter; but I have made no extensive investigation 
of this matter—being content with L.’s own statement. 
> Of. p. 121 et alibi, supra. 
