324 LEEUWENHOEK AND HIS “‘ LITTLE ANIMALS ”’ 
length of about 0°9 mm.]: and its resolution was so good that, 
with suitable illumination, it was capable of resolving the 4th 
croup of lines on a Nobert test-plate (7.e., a scale 10 » long 
subdivided into 7 equal parts by parallel lines ruled with a 
diamond on glass). In Harting’s opinion this was probably 
the optical limit of Leeuwenhoek’s lenses: but the lens which 
he studied was only one of hundreds, and some of the others 
—now lost—may well have been superior.” 
Leeuwenhoek himself has—to my knowledge ’—left us no 
account of his particular procedure in making and mounting 
lenses. Others published their methods,* but he never did. 
How to grind and polish lenses for spectacles and telescopes 
was common property, however, at the time when he wrote; 
and it seems probable that he worked by the ordinary rules 
and with the customary apparatus. If you had asked him 
how to make a very powerful lens, of very short focus, he 
would doubtless have told you that it could only be made by 
the usual methods: but as such a glass would be much 
smaller, and more convex, you would have to do everything 
on a smaller scale, and pay more attention to details. If you 
can make a good lens with a focus of 1 foot, then you can— 
if you take pains, and know your job—make an equally good 
one with a focus of +$5 of an inch. It is more difficult, and 
takes longer; but that is all. It is a question only of the 
time and trouble that you are prepared to expend. The 
general methods of grinding and polishing lenses were no 
secrets: and when Leibniz asked old Leeuwenhoek why he 
did not educate a school of younger men in the art, he made 
the following reply :” 
1 Op. cit., Vol. I, p. 404. 
2 The lens examined by Harting was in a microscope preserved, at that 
date, in the physical collection at Utrecht. The instrument now in the 
zoological collection of the same university is greatly inferior—its lens 
having a focal length of about + in. (fide Mayall, 1886). 
* Crommelin (1929), in his recent admirable essay on lens-grinding [and 
lens-grinders] in the 17th century, is of the same opinion. On this subject, 
he says, L. “has left us entirely in the dark’’. 
* See especially Manzini (1660). 
> Send-brief XVIII, pp. 168-9 [Epist. Physiol. XVIII, p. 167]. Letter 
dated 28 Sept.1715. Not in Phil. Trans. 
