LETTER 19. METHOD OF OBSERVING lg 
flock of sheep. And just as the supposed number may 
differ from the true number by fully 100, 150, or even 200, 
in a flock of 600 sheep, so may I be even more out of my 
reckoning in the case of these very little animalcules: for 
the smallest sort of animalcules,’ which come daily to my 
view, I conceive to be more than 25 times smaller than 
one of those blood-globules which make the blood red 5 
because I judge that if I take the diameter of one of 
these small animalcules as 1, then the diameter of a 
blood-globule is at least 3. 
These, Sir, are the trifling observations which I have 
shown to divers curious persons, to their great satisfaction ; 
but the other things that I have seen, and my particular 
microscope, I cannot yet resolve to make public: which 
I beg you, Sir, and your fellow philosophers, not to take 
amiss. 
Since sending off my letter” concerning the little 
animals in water, I’ve not remained idle; but I have con- 
tinued to examine divers sorts of water, examining even 
that which was distilled or boiled. During the last sharp 
spell of cold, when all the little animals had perished, I let 
the water thaw by the fire; and when it had stood a whole 
day in my bedchamber, with the fire kindled all the time, I 
saw, after the lapse of 24 hours (and at another time after 
17 hours), that living creatures had come again in the 
water. Upon this subject I might, indeed, say something 
further; but I note that my former letter is still under 
your consideration, so I will spare you more. 
To give your Philosophers further assurance, con- 
cerning the reality of the multifarious living creatures in 
even only a very little quantity of water, ‘tis my intention, 
when they appear again in great plenty in the water, to 
obtain testimony thereof, which I shall then send you. 
The foregoing Letter was read at the meeting of the Royal 
Society on 5 April 1677 [O.S.]; and the observations excited 
‘7.e., Bacteria. 
? Referring to Letter 18 (p. 117 swpra). 
