Lectures and collections: Cometa, Microscopium

Hooke, R.
London: J. Martyn

full text facsimile online at Google Books

After Henry Oldenburg died, the Royal Society did not have the resources to replace him as owner, publisher, and editor of Philosophical Transactions. Hooke took it upon himself to publish a dozen letters/articles written by himself and other scholars who had previously published in Philosophical Transactions, including Robert Boyle and Edmond Halley.

The section titled Microscopium opened with Hooke's response to two of Leeuwenhoek's letters. Neither of these letters was accompanied by figures.

Hooke published only a short excerpt from the end of Leeuwenhoek's Letter 21 of October 5, 1677, addressed to Henry Oldenburg before Leeuwenhoek knew that he had died a month earlier, in September. He published a couple of paragraphs from the end of the letter about the quantity of little animals Leeuwenhoek was observing. Hooke's response in Microscopium is found on the page Hooke's Three Tries under Learn More below.

Immediately following his response to Letter 21, Hooke included most of Letter 23 of January 14, 1678, addressed to himself. His response (below) is a long description of microscope construction and use.

Microscopium, pp. 89-104 (my emphasis)

The manner how the said Mr. Leeuwenhoeck doth make these discoveries, he doth as yet not think fit to impart, for reasons best known to himself; and therefore I am not able to acquaint you with what it is: but as to the ways I have made use of, I here freely discover that all such persons as have a desire to make any enquiries in­to Nature this way, may be the better inabled so to do.

First, for the manner of holding the liquor, so as to examine it by the Microscope, I find that the way prescribed by Mr. Leeuwenhoeck is to include the same in a very fine pipe of glass, and then to view it by the help of the Microscope; for by placing that at a due distance, whatever is contained in the said liquor will most easily be discovered: The liquor will most easily insinuate it self into the cavity of the said pipe, if the end thereof only be just put within the liquor. This as it is exceedingly convenient for many trials, so is it not very difficult to prepare; but be­cause every one is not instructed how to proceed in this matter, and it may cause him more trouble than needs to procure them, I will here describe the way; and so much the rather, because the same apparatus will [Page 90] serve for the preparing of Microscopes: as I shall af­terwards shew.

Provide then a box made of tin, with a flat bottom, and upright on all sides; let this have fixed within it to the bottom a small piece of tin, hollowed like a ridg tile, so that the wiek of the Lamp may lie and rest up­on it, and let the Tin-man fix on it a cover of tin, so that there may be only left one part of the aforesaid box open, to wit, where the bent tin piece and the wiek do lie and come above the sides: this cover may be turned back on its hinges when there is occasion to raise the wiek, or put in more oyl, &c. but for the most part ought to lie flat and covered; for whilst it is using, it is necessary to keep the flame from spreading too much, and taking fire all over. This box must stand within another box of tin, made large enough to con­tain it; the use of which is to keep the former Lamp Box from fowling the board or table on which it stands: This stands upon a board about one foot square, into which is fastned a standard or stick upright, cleft so as to pinch and hold the sodering pipe between its clefts, which may be fastned with a screw, or a slipping ring; through which pipe, blowing with your breath, the flame will be darted forward with great swiftness and brightness: if then into this flame you hold a small piece of a glass pipe, made of white glass, (for green glass, or coarser glass will not be melted easily in this flame) and keep it turning round between your fingers and thumbs, you shall find that the flame will in a very short time melt the middle part of the said pipe; so that if you remove it out of the flame, and draw your hands one from another, you may easily draw the for­mer pipe into a very small size, which will yet remain hollow, though drawn never so small. The best Oyl for this purpose is good clean Sallat Oyl, or Oyl Olive; but high rectified Spirit of Wine is yet better, and cleanlier, but much more chargeable; and for most uses the Oyl Olive will serve. This I have set down, be­cause [Page 91] many who are far off in the Country cannot have the convenience of going to a Lamp-blower as oft as they have occasion for such pipes; which if they pro­vide themselves with small white glass pipes from the Potters, they may accommodate themselves withal, though they have nothing but a large candle, and a to­bacco-pipe, instead of the aforesaid apparatus, though not altogether so conveniently. But I would rather advise them to have a Lamp made, which most Tin-men know how to fit and prepare; and so it will not need much more description.

But this way of Mr. Leeuwenhoecks, of holding the li­quors in small glass pipes, though it be exceedingly in­genious, and very convenient for many examinations, yet for divers others 'tis not so well accommodated as this which I contrived my self for my own trials, at least for those Microscopes I make use of; what it may be for those which Mr. Leeuwenhoeck uses I know not.

I take then instead of a glass pipe a very thin plate of Muscovy glass, this serves instead of the moveable plate which is usually put upon the pedestal of Micro­scopes; but because the common pedestal hitherto made use of in Microscopes is generally not so conve­nient for trials of this nature, I lay those by, and in­stead thereof I fix into the bottom of the Tube of the Microscope, a cylindrical rod of Brass or Iron. Upon this a little socket is made to slide to and fro; and by means of a pretty stiff spring, will stand fast in any place. This hath fastned to it a joynted arm of three or four joynts, and at the end a plate about the bigness of a half crown, with a hole in the middle of it about three quarters of an inch wide; upon this plate I lay the Muscovy glass, and upon that I spread a very little of the liquor to be examined; then looking against the flame of a Candle, or a Lamp, or a small reflection of the Sun from a globular body; all such parts of the liquor as have differing refraction will manifestly appear. By this means I examined the water in which [Page 92] I had steeped the pepper I formerly mentioned; and as if I had been looking upon a Sea, I saw infinite of small living Creatures swimming and playing up and down in it, a thing indeed very wonderful to behold.

If the flame of the candle were directly before the Microscope, then all those little Creatures appeared perfectly defin'd by a black line, and the bodies of them somewhat darker than the water; but if the can­dle were removed a little out of the axis of vision, all those little Creatures appeared like so many small pearls, or little bubbles of air, and the liquor in which they swimmed appeared dark; but when the water began to dry off, the bending of the superficies of the liquor over their backs, and over the tops of other small motes which were in the water made a confused appearance, which some not used to these kind of examinations, took to be quite differing things from what they were really; and the appearances here are so very strange, that to one not well accustomed to the phaenomena of fluids of differing figures and refractions, the examina­tions of substances this way will be very apt to mis-in­form, rather than instruct him; especially of such sub­stances as are not perfectly fluid, and will not readily and naturally smooth their own superficies, such as Tallow, concreted Oyls, Marrow, Brains, Fat, inspissa­ted juyces, &c. for if those substances be so examined by spreading them upon this plate, and be looked up­on against the candle, or other small defined light, all the inequalities left on the surface by the spreading do by the refractions of the rays of light render such odd appearances, that they will easily deceive the examina­tor, and make him to conceive that to be in the tex­ture of the part which is really no where but in the make of the superficies of it. This therefore as ano­ther great inconvenience to be met with in Microscopi­cal Observations, I prevent by these ensuing methods: First, all such bodies as Fat, Oyl, Brains, Rhobs, Pus, tough concreted Flegm, and the like, whose surfaces [Page 93] are irregular, and ought to be reduced to smoothness before they can be well examined, I order in this man­ner: First, I provide a very clear and thin piece of looking-glass plate very smooth and plain on both sides, and clean from foulness: upon the surface of this I lay some of those substances I last mentioned, then with such another piece of Looking-glass plate laid upon the said substance I press it so thin as not only to make the surfaces of it very smooth, but also to make the substance of it very thin; because otherwise, if the substance be pretty thick, as suppose as thick as a piece of Venice paper, if it be a whitish substance, the mul­titudes of parts lying one upon another in such a thick­ness, do so confound the sight, that none of them all can be distinctly seen: but if by squeezing the said plates hard, and close together, it be reduced to a twentieth part perhaps of that thickness, the substance may be well looked through, and the constituent parts may be very plainly discovered. Thus also 'tis very visible in the Globules of milk and blood, discovered by the ingenious Mr. Leeuwenhoeck, for when either of those substances are thick, the multitude of those little Glo­bules confound and thicken the liquor so as one can­not perceive any thing until it be run very thin; for then all the remaining Globules with their motions may very distinctly be apprehended. This therefore is an expedient by which thousands of substances may be examined; and therefore the more fit to be communi­cated, that there may be the greater number of obser­vers well accommodated for such trials. These plates therefore may be contrived so as to be pinched toge­ther by the help of screws, and a frame, that thereby they may be forced the closer and the evener together, as there shall be occasion; and may be kept firm and steady in that posture, and then, that it may some ways or other be conveniently fastned to the former plate, so as to be moved this way or that way steadily, as there shall be occasion.

[Page 94] But there are other substances which none of these ways I have yet mentioned will examine, and those are such parts of animal or vegetable bodies as have a peculiar form, figure, or shape, out of which if it be put, the principal thing looked after is destroyed: such are the Nerves, Muscles, Tendons, Ligaments, Membranes, Glandules, Parenchymas, &c. of the bo­dy of Animals, and the Pulps, Piths, Woods, Barks, Leaves, Flowers, &c. of Vegetables. Some of these which are not made by dissection or separation from other parts may be viewed alone; but there are others which cannot be well examined unless they be made to swim in a liquor proper and convenient for them: as for instance, the parts of flesh, muscles and tendons: for if you view the fibres of a muscle encompassed on­ly with the air, you cannot discover the small parts out of which it is made: but if the same be put into a li­quor, as water, or very clear oyl, you may clearly see such a fabrick as is truly very admirable, and such as none hitherto hath discovered that ever I could meet with; of which more hereafter, when I shew the true mechanical fabrick thereof, and what causes its motion. Thus if you view a thred of a Ligament, you shall plainly see it to be made up of an infinite company of exceeding small threads smooth and round, lying close together; each of which threads is not above a four hundredth part of the bigness of a hair: for compa­ring those of Beef with a hair of my head, which was very fine and small, viz. about a 640. part of an inch, I found the Diameter thereof to be more than twenty times the Diameter of these threads; so that no less than 163 millions, besides 840 thousands of these must be in a ligament one inch square. I shall not here en­large upon the admirable contrivance of Nature in this particular, nor say any thing farther of the reason of the greater strength of the same substance drawn into smaller than into greater threads; but only this in ge­neral, that the mechanical operations of these minute [Page 95] bodies are quite differing from those of bodies of great­er bulk, and the want of considering this one thing hath been the cause of very great absurdities in the Hypotheses of some of our more eminent modern Phi­losophers: For he that imagines the actions of these lesser bodies the same with those of the larger and tractable bodies, will indeed make but Ari­stotles wooden hand at best. This put me in mind likewise of advertising the Experimenter that he pro­vide himself with instruments, by which, to stretch and pull in pieces any substance whilst the same is yet in view of the Microscope, of which there may be many which any one will easily contrive, when he hath this hint given him of the usefulness thereof in the examination of the texture of several substances; as of Tendons, Nerves, Muscles, &c. those I have made use of were made to open like a pair of Tobacco Tongues, by two angular plates of thin brass rivetted together, which by pinching the opposite end, would either open or shut at the other, as I had occasion. These having a part extended between the two tops, were fixt at a due distance from the object-glass that the body extended between them might be distinctly seen; then with my finger squeezing together the op­posite ends, the other ends opened, by which means how the parts stretched and shrunk might be plainly discovered. Now as this is of use for some kind of substances, so the two glass plates are for others, and particularly for squeezing of several substances be­tween them, so as to break them in pieces, as those lit­tle Creatures in pepper-water, or the Globules in blood, milk, flegm, &c. whereby the parts within them may yet farther be enquired into, as Mr. Leeuwenhoeck I find hath done by his latest Observations. Whether he makes use of this way, or some other, I know not.

Having thus given a description of the appurtenan­ces, it remains that I come to the description of the Mi­croscope [Page 96] it self, which is the principal instrument, and without which all the rest are insignificant.

The Microscopes then I design here to describe, are only of two kinds, either single or double.

The single Microscope I call that which consisteth only of one glass, though it have a double refracting superficies; and the double one I call that which is compounded of two glasses, though it hath for the most part a quadruple refraction of the Rays.

The single Microscope then consisteth of one small lens so fastened into a cell, that the eye may come con­veniently to look through the middle part or Axis of it; of these there are various sorts, as double Convexes, or plain Convexes, or perfectly spherical.

I shall not need to describe the common lenses which are every where made use of for this purpose, being plano-convexes of Spheres about half an inch Diame­ter, save only this, that 'tis best to turn the plain side towards the object, and the convex to the eye: nor shall I say much concerning those double Convex Glas­ses, there being no great difficulty in the making or using of them; but that the smaller the sphere is in which they are made, the nearer do they bring the ob­ject to the eye; and consequently the more is the object magnified, and the better and truer they are polisht in the Tool, the more clear and distinct doth the object appear, but to make any of a Sphere less than 1/10 of an inch in Diameter is exceeding difficult, by reason that the glass becomes too small to be tractable; and 'tis very difficult to find a cement that will hold it fast whilst it be completed; and when 'tis polisht, 'tis ex­ceeding difficult to handle and put into its cell: be­sides, I have found the use of them offensive to my eye, and to have much strained and weakened the sight, which was the reason why I omitted to make use of them, though in truth they do make the object appear much more clear and distinct, and magnifie as much as the double Microscopes: nay, to those whose eyes can [Page 97] well endure it, 'tis possible with a single Microscope to make discoveries much better than with a double one, because the colours which do much disturb the clear vision in double Microscopes is clearly avoided and pre­vented in the single. The single Microscope therefore which I shall here describe, as it is exceeding easie to make, so is it much more tractable than the double Con­vex glasses made the common way by working them in a hollow Hemisphere with water and sand; for those, supposing them made with all the accurateness imagin­able, will be far short from being so well polisht as these; and wanting the stem or handle which these have, they are infinitely troublesome to remove, or place, or to cleanse when there shall be occasion.

Take then a small rod of the clearest and cleanest glass you can procure, free if possible from blebbs, sands, or veins; then by melting it in the flame of a Lamp made with Spirit of Wine, or the cleanest and purest Sallet Oyl, draw it out into exceeding fine and small threads; then take a small piece of these threads, and in the same flame of the aforesaid Lamp melt the end of it, till you perceive it to run into a little ball or glo­bule of the bigness desired; then suffer it to cool, and handling it by the aforesaid thread of glass, which is as it were a handle to it, fix it with a little wax upon the side of a thin plate of Brass, Silver, or the like, that the middle of it may lie directly over the middle of a small hole pricked through the said thin plate with a needle: then holding this plate close to the eye, look through the said little hole, and thereby you may also see very clearly through the aforesaid Globule, fixed with wax on the side that is from the eye: if then either by a lit­tle joynted arm, or by a little soft wax, and a needle, or a thin plate of Muscovy glass, you fix the object you would examine; so that it may be at a due distance from the said little Globule, you will perceive the minute parts thereof very distinct. The focus of a sphere looked on by the naked eye, is about half the radius of the [Page 98] sphere, without the superficies of it; but this is varied much by the age of the eye that looks through it, by the imagination also of the person, and by the differ­ing specifique refraction of the glass made use of.

By this means I have prodigiously magnified some small bodies, insomuch that I have been able to see and distinguish the particles of bodies, not only a million of times smaller than a visible point, but even to make those visible, whereof a million of millions of them would hardly make the bulk of the smallest visible sand; so prodigiously do these exceeding the little Globules of glass inlarge the prospect of humane sight into the more private recesses of nature.

If the things to be viewed be liquors, they may be included either in those little pipes of Mr Leeuwenhoeck I newly mentioned, or else they may be put upon ex­ceeding thin plates of Muscovy glass or Selenites, and the other side of the plate may be made to touch the Globule, or at least be fixed at such distance, as may make the parts of the liquor distinct: If you make use of a Looking-glass plate to spread the liquor upon you would examine, you may turn the liquor towards the Globule, and you may therein easily see all the parts very distinctly, without at all hurting the prospect by the interposition of the Muscovy glass; which though it be exceeding clear, especially if the plates be very thin, yet hath it some flaws, and some opacousnesses in it, which do somewhat cloud the prospect.

If further, you would have a Microscope with one single refraction, and consequently capable of the great­est clearness and brightness that any one kind of Mi­croscopes can possibly be imagined susceptible of, when you have fixt one of these little Globules as I have di­rected, and spread a little of the liquor upon a piece of Looking-glass plate, then apply the said plate with the liquor, next to the Globule, and gently move it close to the Globule, till the liquor touch; which done, you will find the liquor presently to adhere to the [Page 99] Globule, and still to adhere to it though you move it back again a little; by which means, this liquor being of a specifique refraction, not much differing from glass, the second refraction is quite taken off, and little or none left but that of the convex side of the Globule next the eye; by which means as much of the incon­venience of refraction as is possible is removed, and that by the easiest and most practicable expedient that can be desired. I could add various other ways of making these Globular bodies both of glass and other substances which will yet farther advance our prospect into nature, and are pleasant to admiration; but those I shall yet reserve till I see what effects the publishing of these may produce, and to the end to excite other persons to be inquisitive into this matter: for let me as­sure them, very much more may yet be done by a way I know, than by this I have here published. And I confess I have very often wondered that no farther im­provement had been made of this Principle, since I publisht it in the year 1664. in the 20. page of my Pre­face to Micrographia: for though some other reasons discouraged me from prosecuting those enquiries, yet I hoped that others might long before this have carried it much farther.

The only inconvenience in these kinds of Micro­scopes, is, that the object is necessarily brought so near the glass, that none but such as are transparent, and to be viewed by a through light are capable of examina­tion by them: such therefore are to be examined by the double Microscope; which, as it is abundantly more tractable, so doth it much less strain the eye; and from the easiness of its use, when well fitted, is much more pleasant: and if ordered as it ought, will magnifie as much more than the common ones hitherto made, as those did more than the naked eye.

Both these Microscopes I have directed Mr. Christopher Cock, in Long-Acre, how to prepare, that such as will not trouble themselves in the making of them, may know [Page 100] where to be accommodated with such as are good.

And of the improvement of this kind of Micro­scope, I see no limits, especially as to the augmenting the visible appearance of such objects as are capable of enduring the increase of light; for since 'tis demonstra­ble that light may be augmented upon any one object susceptible to any given degree, and that by the double Microscope the image can be augmented to any assign­ed magnitude, what but the difficulty of making all things correspondent should limit the power of such an instrument. Now the making of this double Micro­scope, though it be somewhat more difficult than of the single one, yet the tractableness thereof when well fit­ted, and its easiness to be cleansed, and applied to use, makes amends for the extraordinary charge, especially the situation of the object; which being capable of any reasonable distance from the object glass, so as to be fit for examination, makes it very desirable. Now as in all other mechanical contrivances, that is best which is plainest, and most simple: so is it in this, wherein nothing more is required, but two plano Con­vex glasses, the one for the object-glass, and the other for the eye-glass: the less the spheres of the glasses be, the more do they magnifie the object; and the thinner and clearer the substance of them be, and the more ex­actly shaped, and the brighter they are polisht, the clearer do they represent it; and the longer the glasses are distant from each other, the more is the image mag­nified, caeteris paribus, though indeed the same thing is performed by glasses of very differing magnitudes, due proportions of all things about it being kept and ob­served. For if as the distance of one object from the object-glass is to the distance of another object from an­other object-glass, so the distance of the first image be to the distance of the second image, the image in both must be equal: if therefore this image be viewed with equal glasses the image must be equally magnified at the bottom of the eye; so that in this way the object is [Page 101] capable of a double way of augmenting, viz. first, the augmenting the figure in the Tube, by the smallness of the object-Glass, and length of the Tube: and second­ly, by the augmenting that image in the bottom of the eye, and that is by the Eye-glass; give therefore light enough to the object, and you may increase the image at the bottom of the eye to what proportion you shall desire. And by a way I shall shortly shew, the objects may be perceived distinct, defined, and colourless, as if seen by the naked eye. In all these ways the manner of applying the light is very significant, and provided it be very strong, the smaller the point be it proceedeth from, the more distinctly doth it exhibit the difference of refraction in the transparent bodies viewed by it, and the plainer will their parts be discovered: The light therefore of the Sun either reflected from a Spherical Convex body, or Spherical Concave body, the object being placed beyond the focus, or Refracted through a Concave or through a Convex, if the object be placed beyond the focus, do exceedingly well. But these with the help of a dark Room do yet better, the object being placed in a Table against the Light, and all other Light screen'd from the Eye by the Dark Room. Much the same thing is done by the Light of a Lamp or Candle in the Night, which is indeed the most conve­nient Light, where Colour is not so much looked after.

Whilest this Discourse was Printing I casually met with a Treatise of P. Cherubine, Printed at Paris, 1677. Entituled, LAVISION PERFAITE, ou les concours des deux axes de la Vision en un sceul point de l' object; Wherein the Author pretends amongst other things to have promoted Microscopes extreamly by so joyning two together, as through them to see the same object distinct with both the Eyes at once, and to see a large object all at one view, by which he af­firms to have discovered some mistakes and untruths in some of those figures I have formerly published in my Micrography. But if he had pleased to have read [Page 102] the Description as well as looked on the Figure, he might have been better informed than by his Preface he would seem to be. I deny not but that there are many failures in some of those draughts, some of my own and some of the gravers committing. Humanum est. But those which he charges for such are not, as he might have seen if he had made use of better glasses than those which he describes, for they are so far short of equalling those I use, that I can demonstrate from his own De­scription of them, that those I made use of did magnifie 10000 times more than that with which he pretends to have made these great Discoveries. Nor is it any thing more than common to see as large an Area as he men­tions, with a glass that magnifies no more than his doth. But I could have shewed him how he might see the whole Creature at once, and yet much more magnifie than that which I have described, nay though the Creature were twice as big, and that with one Eye only, which is much to be preferred before that with two. However I should be very glad to hear what Discoveries he doth make with his binocular Micro­scope more than was seen before. As also that he would please to demonstrate the truth of Parallelogram pre­scribed for certain uses, pag. of Dioptrique Ocu­laire, and in the Fourth Chapter of the Fourth Part of this Book. But to digress no farther from what I was describing. I must add that with both these kinds of Mi­croscopes have I examined several substances; as par­ticularly the steepings of several grains and seeds in rain-water. And though I have not yet found any one tincture more prolific than this of Pepper; yet 'tis not the only tincture in which they do both breed and in­crease. I have seen several sorts in the steeping of Wheat, Barly, Oats, Coffee, Anniseeds, Pease, &c. some not above a third part of a hair in thickness; others not above the twentieth part of the breadth of a hair, and some not more than a thirtieth part of that breadth; so that no less than 900 of these least must go to make an [Page 103] area as big as that of an hair cut transversly, and 27000 to make a Cylinder as big as the hair of ones head, and of equal height with the Diameter of that hair, which one may just call a visible point, and no more; few eyes seeing things distinctly much smaller: Now the Diameter of a hair of my head being by examination found but the 640 part of an inch, it follows that no less than 19200 or to use a round summ about 20000 of them may lie in the length of an inch, and consequent­ly, that a circle an inch Diameter will be to the area of one of these cut transversly as 400000000 to 1. four hundred millions to 1 and a Cylinder one inch Dia­meter, and one inch high, will be to one of these mites, as 8000000000000 to one, eight millions of millions to one. If therefore we compare the magnitude of one of these animals to the magnitude of other creatures living in the water, we shall find that these will be found much smaller in comparison to the body of an ordinary Whale, than the body of the same Whale will be to the body of the whole Earth; which may prove an argument for an anima mundi perhaps to some. But let every one make his own inferences, and believe his own eyes, for they will make the best impression on his reason and belief. Now if the Creature be so ex­ceeding small, what must we think of the Muscles, Joynts, Bones, Shells, &c. certain it is, that the Me­chanism by which Nature performs the muscular moti­on is exceedingly small and curious, and to the perfor­mance of every muscular motion in greater Animals at least, there are not fewer distinct parts concerned than many millions of millions, and these visible, as I shall hereafter shew through a Microscope; and those that conceive in the body of a muscle, little more curiosity of mechanism than in a rope of the same bigness, have a very rude and false notion of it; and no wonder if they have recourse to Spirits to make out the Phaeno­mena: but of this hereafter more.

Further, I have examined the constitution of Blood, [Page 104] Milk, Flegm, &c. and found them much the same with what Mr. Leeuwenhock has declared. A little fat laid upon the glass plate whilst warm, melts, and becomes transparent, but observed in a convenient posture a­gainst the light of a candle, &c. till it congeals, and shrinks, make a pleasant fluid, and shews how consi­derably a fluid and solid body do vary, and may give us a good hint to conjecture at the reason of the swel­ling and greater lightness of Ice than of Water. The first beginnings also of the shooting or crystallising of Sugar into rectangular parallelipipeds, Alum, Salt, Vi­triol, &c. are strangely surprizing and instructive, I could enumerate multitudes of these.


This publication contains the only contemporary printed versions of two Leeuwenhoek letters. They are listed separately among the primary printed sources. The rest of the book contains articles by Hooke relevant in their own right and Hooke's comments on Leeuwenhoek's letters, so the book is listed among the secondary printed literature.