LEEUWENHOEK’S MICROMETRY RB 
A Hair on a Louse is a less accurate comparison: for the 
setae on P. corporis vary greatly in diameter, in different parts 
of the body. Moreover, they all taper to a point, so that their 
sides are not parallel. Large hairs, I find, have a maximum 
width of about 4 »: but smaller ones measure only 2°5 w or 
even less. “The thickness of the hairs wherewith the body 
of a louse is beset” I estimate—very roughly—to be something 
more than 3 yp. 
I can add little else to what Leeuwenhoek himself tells us 
about his micrometry.' I may mention in conclusion, how- 
ever, that he had some correspondence on the subject with 
Dr James Jurin in the last year of his life. Jurin, in 1718, 
invented” a new method of measuring small objects ; and in 
1722 he wrote to Leeuwenhoek about it, and persuaded him 
to try it. Leeuwenhoek did so, and his answering letter was 
published.’ Though it is not generally known, Jurin’s own 
draft (in English) of his letter to Leeuwenhoek is still extant. 
It is an interesting letter, and its present owner* has kindly 
allowed me to read and copy it: but asit has no bearing upon 
Leeuwenhoek’s own methods, I need not print it here. 
Finally, I may note that the we:ghts mentioned by 
Leeuwenhoek, in various letters, sometimes furnish clues to 
his measurements. We could, indeed, exactly determine some 
of his small measures of length if we knew the exact equivalents 
of his weights in modern units. We do not know, however, 
how closely his “grain” or “ounce” agree with modern 
weights of the same name; though fortunately he recorded his 
1 Tt should be noted that an earlier attempt to evaluate L.’s measure- 
ments was made by Muys (1741; p.332, note 72). His words are worth 
consideration ; but his estimates can hardly be regarded as satisfactory at 
the present time, so I shall not discuss them here. 
2 See Jurin (1718). His method was simple and ingenious. It consisted 
in closely winding fine hairs or silver wire on a needle, along a measurable 
length, and then determining the diameter of the hair (or wire) by counting 
the number of turns in that length. The diameter of the filament being 
thus ascertained, it was cut into small bits and strewn among the objects to 
be measured, whose size was estimated (under the microscope) by comparison. 
In this way Jurin determined the diameter of a human red blood-corpuscle 
to be gz'z5 inch (a very close approximation). 
3 Phil. Trans. (1723), Vol. XXXII, No. 377, p.341 (MS.Roy.Soc. 
19 March 1723). 
4 Mr A. K. Totton, a kinsman of Jurin and a former pupil of mine. 
22, 
