James Jurin wrote (AB 370) to Leeuwenhoek about how to measure his microscopic observations

December 24, 1722

Jurin thanked Leeuwenhoek for his most recent letter on diamonds and especially for translating his letters into Latin. He enclosed a wire that was exactly "1/485 part of ye London inch" along with a detailed method for using the to measure microscopic objects such as blood globules.


Wellcome MS 6143

Worthy Sir,

The particular honour you did me in your last Letter, containing your curious Observations upon Diamonds, was extremely agreeable to me upon many accounts. But especially I return you my most humble thanks for your great condescension in y" regard you have shewn to my desire of receiving your Letters in Latin. You have thereby freed me from ye apprehension which before I was often under, of not taking the true sense of some parts of your Letters, & it is wth great pleasure, that I see your discoveries in this Letter express'd wth a perspicuity & an elegance worthy of ye Subject, & wch could hardly be expected in any translation made here, because we could not have ye opportunity of consulting you, in case of ambiguity. I had ye pleasure of entertaining ye Royal Society wth ye recital of it, some days ago, & am commanded by them to return you their thanks and acknowledgement for ye many curious & accurate Observations contain'd both in this & your Letters of June 13th & July 7th, the translation of wch I had communicated to them sometime before. The former of these seems to me to sapp ye very foundations of ye System of Generation by means of the Ovarium.

I take, Sir, ye liberty of communicating to you a Method, for measuring ye diameters of very minute Objects, as ye Blood Globules &c wch I happen'd upon some years ago, on occasion of reading some of your Observations, in wch you compare ye magnitude of Microscopical ObJects wth ye Diameter of a hair of a Man's Beard, or a grain of Sand; wch generally serves very well to give us a notion of ye minuteness of ye body you observe. But for some particular very nice Observations, where one is desirous of knowing very nearly ye exact magnitude of an Object, this Method is insufficient because ye hairs of ye beard or head, not only of different Persons, but even of ye same Persons, differ very much in diameter one from another; & ye same may be said of grains of Sand, as no body knows better than your self. I choose therefore to make use of fine Silver wire for this purpose, of wh I cut off a smale piece & lay it upon ye plate of ye microscope, moving it wth ye point of a pin or needle, till it lies in a proper situation to compare its Diameter wth that of ye Object I would measure. But to know ye the diameter of my Wire, I take my small Cylindrical body, as for instance one of ye long Needles, that Women use in knitting, & I wind my Wire a great number of times round it, taking care from time to time to push ye several rounds of ye wire close to one another wth my nail, & observing wth ye help of an ordinary microscope that ye rounds lie close together, & that none of them lies over ye rest. When I have got a sufficient number of these upon my needle, as for instance enough to make half an inch or an whole inch, having all along kept account of y" number of rounds I have laid upon y" needle, I know thereby how many rounds, or how many diameters of my wire make an inch in length, & consequently what part of an inch is equal to ye Diameter of my Wire. But to prevent any mistake I take care to count ye number of rounds again, as I unwind ye wire from off ye needle. When this is once done, my wire serves me as a basis for all ye Observations I ever make of this sort.

I send you enclosed a small quantity of a wire I measured after this manner, whose diameter I found to 1/485 part of ye London inch. I have observ'd yt four of ye globules of my blood lying together are nearly equal to one diameter of this wire, & consequently, that ye Dr [diameter] of a Blood globule is nearly equal to 1/2000 part of an Inch. You will do me a particular pleasure if you will be pleased to repeat this Experiment wth ye Wire I send you, & not only upon your own blood, but that of other Persons in sickness & in health & likewise ye blood globules of other Animals. I had formerly made ye same Observation, but in a grosser manner, & had estimated the dir [diameter] of a blood globule to be 1/3240 of an Inch, as may be seen in Phil. Trans. No 355.

It is not impossible that a number of Observations of this kind may give us some farther light into the alterations made in ye blood by diseases, at least it will either shew ye falsity, or confirm ye truth of a notion entertain'd by some Physicians of ye breaking, or dividing ye Blood globules in Fevers, & other diseases, especially when ye Blood ... of its self makes its way and breaks out at uncommon passages. I am, wth great Esteem, Honour'd Sir,

Your most obliged and most humble Servant,
J. Jurin, R.S. Secr.