90 LEEUWENHOEK AND HIS “‘ LITTLE ANIMALS”’ 
1723, after discussing the dimensions of blood-corpuscles, he 
wrote : 
This also I have to add: that notwithstanding my 
advanced age cannot but hinder my sight, yet my right 
eye, to my great annoyance, groweth somewhat dim. I 
think this comes to pass for the reason that sundry 
blood-corpuscles, floating in the crystalline humor,’ stray 
in front of my sight; some whereof being joined together 
in no or only an irregular order, with others floating 
separately, cause an appearance of little clouds* in my 
eye. Moreover, as I generally use my right eye, I 
readily shut my left eye whilst making observations, 
wherefore my eyesight is dimmer than ’twas wont to be. 
Not so long ago, to wit this last January, I was seized 
with a violent motion about that great and necessary 
organ we call the diaphragm: indeed, ’twas such that 
those present were not a little alarmed at it. When the 
motion abated, and I sought to know the name of this 
distemper, the Physician who was at hand replied that 
it was a palpitation of the heart. But I thought the 
*To Dr James Jurin, Sec. R.S. MS.Roy.Soc. Printed in Phil. Trans. 
(1723), Vol. XXXII, No. 877, p.341; not printed in Dutch or Latin 
collected works. I translate from the original Latin MS.—inscribed in an 
unknown hand (not Hoogyliet’s). 
? humor crystallinws—the old name for what is now called the “ crystalline 
lens” (cf. Blankaart, 1748, swb voce). This structure was described in 
detail, in many animals, by L.; and he knew that it was solid and stratified 
and that it contained no blood-vessels or corpuscles. Consequently, I 
cannot help thinking that in his original words—here mistranslated (?) into 
Latin—he must really have referred to the vitreows humor: which would 
obviously be nearer the truth. 
* mibeculae Phil. Trans. mnubeculae MS. The printed word is meaning- 
less, and greatly puzzled me until I discovered that it was merely a 
misreading of the original. lL. was here referring, of course, to the familiar 
entoptic phenomena called muscae volitantes—now known to arise from 
shadows cast on the retina by strands in the vitreous humor. (Cf. Brewerton, 
1930.) These appearances were first described by L. in his Letter 41, 
14 Apr. 1684, to F. Aston (Ondervind. & Beschouw., 1684, p.19; also in 
Latin editions; incomplete English translations in Phil. Trans. XIV, 790 
[ =780], and Hoole, I, 241). 
