James Jurin wrote Letter L-582 to Leeuwenhoek about how to measure his microscopic observations

January 4, 1723
Collected Letters volume: 

The date is New Style, which was eleven days ahead of the Old Style date of 24 December 1722 that Jurin used in London.

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.


A signed autograph copy of the letter, with Jurin’s signature, is found in London, Wellcome Library, MS. 6143, 5 pp. Copied and pasted from Collected Letters, vol. 20, "this volume" in the footnotes. The spelling has been modernized.

24 Dec. 1722

Worthy sir[1],

The particular honour you did me in your last letter, containing your curious observations upon diamonds[2], was extremely agreeable to me upon many accounts. But especially I return you my most humble thanks for your great condescension in the regard you have shown to my desire of receiving your letters in Latin[3]. You have thereby freed me from the apprehension which before I was often under, of not taking the true sense of some parts of your letters, and it is with great pleasure, that I see your discoveries in this letter expressed with a perspicuity and an elegance worthy of the subject, and which could hardly be expected in any translation made here, because we could not have the opportunity of consulting you, in case of ambiguity.

I had the pleasure of entertaining the Royal Society with the recital of it, some days ago[4], and am commanded by them to return you their thanks and acknowledgement for the many curious and accurate observations contained both in this and your letters of June 13th and July 7th, the translation of which I had communicated to them sometime before[5]. The former of these seems to me to sap the very foundations of the system of generation by means of the ovarium.

I take, sir, the liberty of communicating to you a method for measuring the diameters of very minute objects, as the blood globules, and which I happened upon some years ago, on occasion of reading some of your observations, in which you compare the magnitude of microscopical objects with the diameter of a hair of a man’s beard, or a grain of sand[6]; which generally serves very well to give us a notion of the minuteness of the body you observe. But for some particular very nice observations, where one is desirous of knowing very nearly the exact magnitude of an object, this method is insufficient because the hairs of the beard or head, not only of different persons, but even of the same persons, differ very much in diameter one from another; and the same may be said of grains of sand, as nobody knows better than yourself. I choose therefore to make use of fine silver wire for this purpose, of which I cut off a small piece and lay it upon the plate of the microscope[7], moving it with the point of a pin or needle, till it lies in a proper situation to compare its diameter with that of the object I would measure. But to know the diameter of my wire, I take any small cylindrical body, as for instance one of the long needles that women use in knitting, and I wind my wire a great number of times round it, taking care from time to time to push the several rounds of the wire close to one another with my nail, and observing with the help of an ordinary microscope that the rounds lie close together, and that none of them lies over the rest. When I have got a sufficient number of these upon my needle, as for instance enough to make half an inch or a whole inch, having all along kept account of the number of rounds I have laid upon the needle, I know thereby how many rounds, or how many diameters of my wire make an inch in length, and consequently what part of an inch is equal to the diameter of my wire. But to prevent any mistake, I take care to count the number of rounds again, as I unwind the wire from off the needle. When this is once done, my wire serves me as a basis for all the 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 the London inch[8]. I have observed that four of the globules of my blood lying together are nearly equal to one diameter of this wire, and consequently, that the dr [diameter] of a blood globule is nearly equal to 112,000 parts of an inch. You will do me a particular pleasure if you will be pleased to the repeat this experiment with the wire I send you, and not only upon your own blood, but that of other persons in sickness and in health and likewise the blood globules of other animals[9]. I had formerly made the same observation, but in a grosser manner, and had estimated the dr [diameter] of a blood globule to be 1/3240 of an inch, as may be seen in Phil. Trans. No. 355[10].

It is not impossible that a number of observations of this kind may give us some farther light into the alterations made in the blood by diseases, at least it will either show the falsity, or confirm the truth of a notion entertained by some physicians of the breaking, or dividing the blood globules in fevers, and other diseases, especially when the blood ... of its self makes its way and breaks out at uncommon passages[11].

I am, with great esteem, honoured sir,
Your most obliged and most humble servant,

J. Jurin,
R.S. Secr.


[1]   Jurin’s previous letter to L. is Letter L-580 of 12 October 1722 (dated 1 October 1722 O.S.), in this volume.

[2]   Letter L-581 of 20 November 1722, in this volume.

[3]   In Letter L-575 of 26 May 1722 (dated 15 May 1722 O.S.), Jurin asks L. to have his letters translated before sending them so that the translator would be able to directly ask L. to clarify any ambiguous wording.

[4]   Letter L-581 of 20 November 1722 was read at the 20 December 1722 O.S. meeting of the Royal Society.

[5]   Dr. Sprengel translated Letter L-576 of 13 June 1722; it was read at the meeting of 8 November 1722 O.S. John Chamberlayne translated Letter L-578 of 7 July 1722; it was read at the meeting of 15 November 1722 O.S. Both letters are in this volume. See Royal Society, Journal Book Original, vol. 13.

[6]   As L. used them, a hair of a man’s beard is 60-80 μ and a grain of common sand is approximately 0.064 mm3.

[7]   In contrast to L.’s single-lens microscope, where the specimen moved and the lens stayed still, Jurin used a conventional double-lens barrel microscope that held the specimen still on a plate while the lenses moved above it.

[8]   A London inch is one present-day inch, as opposed to the inch measurements used at the time elsewhere in Europe, which were a little longer than one inch.

[9]   L. followed Jurin’s request and reported his results in Letter L-584 of 19 March 1723, in this volume. He too found that the width of four red blood cells equaled the diameter of Jurin’s wire.

[10] Jurin is mistaken about the number. Philosophical Transactions no. 360 has an article by him titled, “An account of some experiments relating to the specifick gravity of human blood.” It begins, “It is well known from the observations of Mr Leeuwenhoek and others, that human blood consists of red globular particles, swimming in a pellucid lympha, or serum.”

[11] Jurin’s next and last letter to L. is Letter L-586 of 6 July 1723 (dated 25 June 1723 O.S.), in this volume.