LETTER 33. 12 NOVEMBER 1680 201 
to divers distinguished Gentlemen, and I make bold to say, 
that people who say such things have not yet advanced so 
far as to be able to carry out good observations. 
For my own part, I can say with truth, that the smallest 
sort of which I shall here speak, I see alive and exhibit as 
plainly to my eye as one sees, with the naked eye, little 
flies or gnats sporting in the air, though they may be 
more than a hundred million times less than a coarse 
grain of sand ; for not only do I observe their progression, 
both when they hurry, and when they slow down, but 
I see them turn about, and stand still, and in the end 
even die; and those that are of a bigger sort,’ I can also 
see running along as plain as you see mice before 
your naked eye; nay, in some I can even see the inward 
parts of their mouth, as they stick them in and out, and 
make play with them; indeed, in one sort I see the very 
hairs on their mouth, though they themselves are several 
thousand times less than a sand-grain. 
As they’ll say ’tis not credible that so great a many of 
these little animalcules can be comprehended in the 
compass of a sand-grain, as I have said, and that I can 
make no calculation of this matter,” I have figured out 
their proportions thus, in order to exhibit them yet more 
clearly to the eye: Let me suppose, for example, that 
I see a sand-grain but as big as the spherical body ABGC 
[Text-fig. 3, p. 202] and that I see, besides, a little animal 
as big as D, swimming, or running on the sand-grain; and 
measuring it by my eye, I judge the axis of ° the little 
animal D to be the twelfth part of the axis of the 
supposed sand-grain, AG ; consequently, according to the 
" From the descriptive details which follow, L. evidently refers here to 
ciliates. 
* The beginning of this sentence—up to the point here marked—is worded 
differently in the MS. and in the printed version. The meaning is, however, 
identical in the two. 
* van (of) is here omitted from the printed version—by a misprint—but 
is present in the MS. 
