APPENDIX TO LETTER 18 165 
Leeuwenhoek here says that he can recognize all the animal- 
cules described by Christiaan, and notes where he had himself 
observed them previously. But he makes one obvious though 
natural mistake: he takes Huygens’s Astasia for the Dileptus 
which he himself discovered, and on which he had observed 
the cilia. Apart from this, Leeuwenhoek interpreted Huygens’s 
protozoa conformably with my interpretations of his own. 
Huygens observed not only the protozoa which Leeuwenhoek 
discovered in pepper-water, but he also saw and delineated 
(his fig. F.) the long thread-bacteria so commonly seen in 
infusions. On these organisms Leeuwenhoek comments : 
These long eels I have seen too. My wonder at these 
animals, was because one was 3 or 4 times as long as 
t’other, yet they were always of the very same thickness ; 
and besides, they swam as well backwards as forwards, 
without my being able to make out any head, or anything 
that looked like a head. I have already written about all 
this to the Royal Society at London’. : 
This is an important passage, as it confirms what Leeuwen- 
hoek had previously said about bacteria. A little later he 
adds the following remarks, with which we may conclude this 
first chapter in Protozoology : 
All these animalcules aforesaid I found too in ordinary 
water, though not so many by a long way as in pepper- 
water. And in the summer, when I feel disposed to look 
at all manner of little animals, I just take the water that 
has been standing a few days in the leaden gutter up on 
my roof, or the water out of stagnant shallow ditches: 
and in this I discover marvellous creatures. 
And whether I put in the water whole white pepper, 
black pepper, coarse pounded pepper, or pepper pounded 
as fine as flour, animalcules always turn up in it, even on 
the coldest days in winter, provided only that the water 
doesn’t get frozen. 
This day [26 December 1678] there are in my pepper- 
water some animalcules which I judge to be quite 8 times 
* See p. 140 above. 
