LETTER 18. RAIN-WATER 121 
The fourth sort! of animalcules, which I also saw 
a-moving, were so small, that for my part I can’t assign 
any figure to em. These little animals were more than 
a thousand times less than the eye of a full-grown 
louse? (for I judge the diameter of the louse’s eye to be 
more than ten times as long as that of the said creature), 
and they surpassed in quickness the animalcules already 
spoken of. I have divers times seen them standing still, 
as ’twere, in one spot, and twirling themselves round 
with a swiftness such as you see in a whip-top a-spinning 
before your eye*; and then again they had a circular 
motion, the circumference whereof was no bigger than 
that of a small sand-grain; and anon they would go 
straight ahead, or their course would be crooked." 
Furthermore, I also discovered sundry other sorts of 
little animals; but these were very big, some as large as 
the little mites on the rind of cheese, others bigger and 
very monstrous.’ But I intend not to specify them ; and 
1 Probably—from the ensuing description—a species of Monas. Certainly 
not bacteria of any kind. 
2 This makes the diameter of the protozoon here described about 6-8 p, 
and is agreeable with its interpretation as Monas vulgaris. 
? If the description applies to Monas—as I strongly suspect—then the 
“spinning” here described was an illusion. I fancy L. saw a Monas attached 
by its caudal filament, and mistook the swirl of the water at its anterior 
end (occasioned by the movements of the small accessory flagellum) for a 
motion caused by the rotation of the body as a whole. 
4 on dan weder soo regt wijt, als crom gebogen MS. These words are hard 
to understand. The above seems to me to be L.’s meaning : but Oldenburg 
translates ‘and then extending themselves streight forward, and by and by 
lying in a bending posture ”’ (Phil. Trans.). It is hardly likely that L. could 
have observed “a bending posture”’ in an organism so small that he could 
discern “ no figure” in it: and as the “circular motion” just mentioned 
evidently refers to the orbit described by the organism—not to the 
animalcule itself—I imagine that ‘‘regt wijt’” and “‘ crom gebogen” likewise 
refer to the path traversed. I should point out, however, that L. elsewhere 
(Letter 38) applies precisely the same words to the shape of the spermatozoa 
of a frog. 
® Some of these were doubtless protozoa, but the “ monsters’ were 
perhaps rotifers. Much later, when describing these animals, L. mentions 
that he had previously discovered them in rain-water, in which he had 
steeped pepper and ginger. See Letter 144, 9 Feb. 1702. 
