Thomas Molyneaux wrote Letter L-155 of 1685-02-13 to Francis Aston about his visit to Leeuwenhoek's house

February 13, 1685

Thomas Molyneaux (1661-1733), younger brother of William Molyneaux (1656-1698), Irish natural philosopher and member of the Royal Society, was finishing his medical training in Leiden when he wrote this report to Aston. His brother was an ally of Philosophical Transactions editor Edmond Halley. The letter, dated in Leiden on 13 February 1685 N.S. was read at the meeting of the Royal Society on 11 February 1685 O.S. After that, there would not be another letter by Leeuwenhoek published in Philosophical Transactions until eight years later, in early 1693.

A copy of the letter is to be found in the Letter Book Original 10.1, 3 pages, titled, “Mr. Tho. Molyneux to Mr. Aston being his Account of Mr. Leewenhoeck, Microscopes, etc. And a further Account of the Produgiouus Os Frontis in the Medicine School at Leyden.” The spelling has been modernized.

Halley and Thomas’s brother William must have been pleased with Thomas’s report. On November 3, 1686, just twenty-five years old, Thomas was elected a fellow of the Royal Society.

The rest of Molyneaux’s letter was published separately as “Part of a letter from Mr. Thomas Molyneux concerning a prodigious os frontis in the medicine school at Leyden. Dec. 29th. 1684. and Febr. 13th 1684/5”, Philosophical Transactions, vol. 15, no. 168, pp. 880-81.

The following month, on 16 March 1685, Molyneaux wrote another letter to Aston (Royal Society, Early Letters, M.1.104), listing the natural curiosities preserved by Dr. Paul Herman and discussing Johannes Swammerdam’s collection of rarities and Leeuwenhoek’s microscopes. He also noted a madman at Harlem fasting 40 days and nights. About L., he wrote,

The glasses Mr Lewenhoeck showed me magnified objects no more than several other glasses I have seen before, and therefore discover nothing but what may easily be seen by help of other microscopes, so an account of them would be no ways satisfactory; ’tis only his own private glasses which make those more than ordinary discoveries. I never heard he sold those glasses of his more common sort.


Dobell gives the text of the original letter on p. 57. Birch's History, v. 4, p 365, has the version below, the spelling and punctuation copyedited by him. The original manuscript is in the Royal Society archives no. 2445, Early Letters, M.1.103, 4 pages and a copy is in the Letter Book Original vol. 10 (1685-1686) p. 1.

I have hitherto delayed answering your last[1], because I could not give you an account of Myn Heer Leeuwenhoeck, but last week I was to wait upon him in your name[2]: he showed me several things through his microscopes, which ’tis in vain to mention here, since he himself has sent you all their descriptions at large.

As to his microscopes themselves, those, which he showed me, in number at least a dozen, were all of one sort, consisting only of one small glass, ground, (this I mention because ’tis generally thought his microscopes are blown at a lamp, those I saw, I am sure, are not) placed between two thin flat plates of brass, about an inch broad, and an inch and 1/2 long; in these two plates there were two apertures, one before, the other behind the glass, which were larger or smaller, as the glass was more or less convex, or as it magnified. Just opposite to these apertures on one side was placed sometimes a needle, sometimes a slender flat body of glass or opaque matter, as the occasion required, upon which, or to its apex, he fixes whatever object he has to look upon; then holding it up against the light, by help of two small screws, he places it just in the focus of his glass, and then makes his observations[3].

Such were the microscopes, which I saw, and these are they he shows to the curious that come and visit him; but besides these, he told me he had another sort, which no man living had looked through setting aside himself; these he reserves for his own private observations wholly, and he assured me they performed far beyond any, that he had showed me yet; but would not allow me a sight of them, so all I can do is barely to believe, for I can plead no experience in the matter.

As for the microscopes I looked through, they do not magnify much, if anything, more than several glasses I have seen, both in England and Ireland: but in one particular, I must needs say, they far surpass them all, that is in their extreme clearness, and their representing all objects so extrordinary distinctly. For I remember we were in a dark room with only one window[4], and the sun too, was then off of that, yet the objects appeared more fair and clear, then any I have seen through microscopes, though the sun shone full upon them, or though they received more than ordinary light by help of reflective specula or otherwise: So that I imagine ’tis chiefly, if not alone in this particular, that his glasses exceeds all others, which generally the more they magnify the more obscure they represent the object; and his only secret[5] I believe, is making clearer glasses, and giving them a better polish than others can do.

I found him a very civil complaisant man, and doubtless of great natural abilities; but, contrary to my expectations, quite a stranger to letters, master neither of Latin, French or English, or any other of the modern tongues besides his own, which is a great hindrance to him in his reasonings upon his observations, for being ignorant of all other men’s thoughts, he is wholly trusting to his own, which, I observe, now and then lead him into extravagancies, and suggest very odd accounts of things, nay, sometimes such, as are wholy irreconsilable with all truth[6]. You see, sir, how freely I give you my thoughts of him, because you desired it.


[1] The letter from Aston requesting that Molyneaux visit L. is not found. However, it must have been written after Molyneaux’s previous letter to Aston of 29 December 1684, a copy of which is to be found in London, Royal Society, Letter Book Original 9.139, p. 331.

[2] “Last week” places Molyneaux’s visit to Delft between Monday, 5 February 1685 and Friday, 9 February 1685 N.S.

[3] See Zuidervaart & Anderson, “Antony van Leeuwenhoek’s microscopes” and Anderson, Lens on Leeuwenhoek,

[4] L. entertained most visitors in the front room on the ground floor of his house. It had two windows. Only rarely did visitors get to the upper room, what L. called his comptoire, which had only one window. For a detailed discussion of L.’s house, see

[5] In “Little Animals”, pp. 330-332, Dobell gives these secret methods several pages of guarded speculation. “All the evidence indicates that it was the method of using this apparatus which he ‘kept for himself alone’: his secret lay, as he tells us repeatedly, in his ‘particular method of observing’”.

[6] The condescending young Molyneaux came from higher social class than L. As part of a wealthy and distinguished Irish family, he had grown up in Castle Dillon, County Armagh, a very large country estate that dwarfed the Gulden Hoofd where L. lived and worked. Molyneaux’s medical education was conducted in Latin and he knew English and French. L. knew only Dutch, so perhaps Molyneaux had brought a friend from Leiden who knew Dutch.