The 1661 Technique of “Glass Drops”, a Possible Link to Bead Lens Microscopes

Robert Hooke published a very brief account of the technique for the making of bead lenses, by melting little threads of glass until they balled up. Such minute glass balls could then be ground into spherical lenses which required very short focal lengths, but in the hands of a master such as van Leeuwenhoek, produced tremendous magnification and clarity. This is what Hooke wrote in his Micrographia (1665):

And hence it is, that if you take a very clear a broken Venice Glas, and in a Lamp draw it out into very small hairs or threads, then holding the ends of these threads in the flame, till they melt and run into a small round Globul, or drop, which will hang at the end of a thread; and further if you stick several of these upon the end of a stick with a little sealing Wax, so that the threads stand upwards, and then on a Whetstone first grind off a good part of them, and afterward on a smooth Metal plate, with a little Tripoly, rub them till they come to be very smooth; if one of these be fixt with a little soft Wax against a small needle hole, prick’d through a thin plate of Bras, Lead, Pewter, or any other Metal, and an Object placed very near, be look’d at through it, it will both magnifie and make some Objects more distinct then any of the great Microscopes. But because these, though exceedingly easily made, are yet very troublesome to be us’d, because of their smallness, and the nearness of the Object; therefore to prevent both these, and yet have only two Refractions, I provided me a tube of Bras…

When tracking down the possible techniques in microscope construction Spinoza may have pursued, it seems that such tiny spherical lenses may have a possible design that Spinoza used. In support of this a Spinoza correspondent and, it seems, collaborator, Johannes Hudde had practiced the single lens, bead technique as early as 1663, and was an advocate to the technique to Moncoyns and then Huygens. It would seem certainly that Spinoza had heard of it, and because of its ease, one may surmise that he at least tried it out. And given Spinoza’s championing of smaller objective lenses for microscopes in his informal debates with Huygens, one would think that he had some experimental success.

As far as I can tell little is known of how this technique came to Western Europe, one supposed from Italy. Robert Hooke had it by 1664 and Hudde and Vossius had something like it in 1663. A piece in this development may though have been published in 1662, in an English translation of selections of Antonio Neri’s L’Arte Vetraria, The Art of Glass. (Interestingly Spinoza died with a 1668 edition of Neri’s book.) Published in the 1662 English edition is an account of the Royal Society experiments with "Glass Drops", a technique brought to Charles II after the Restoration by the excentric and sometimes brilliant German prince Rupert who became a member of Charles’s privy council.

As one can see from the below, the method of forming drops straight from the pot to be cooled in various liquids was NOT the one used by Robert Hooke a couple of years later, but one can perhaps consider that the production of these very fine threads at the end of small dollops of glass may be related to the development of the single lens thread-melting technique (perhaps in conjunction with other sources). I post below a transcription of the original text as a resource for the study of the development of the bead lens technique. I have not read further accounts of this testing, nor have I invested the time to look at other Royal Society experiments during this time.

An Account of the Glass Drops.

Here Drops were first brought into England by his Highness Prince Rupert out of Germany and shewed to his Majesty, who communicated them to His Society at Gresham College. A Committee was appointed by the Society, who gave this following Account of them, as ’tis Registred in the Book appointed for that purpose, and thence transcribed by their permission, and there published. The which I the rather desired, that this might be a pattern for experiments to be made in any kinde whatsoever, as being done with exceeding exactness.

This account was given to the Society by Sir Robert Morray. MDCLXI.

A B the thread, B C the body, B the neck, A the point or end of the thread.

They are made of Green-glass well-refined; til the Metall (as they call it) be well refined, they do not at all succeed, but crack and break, soon after they are drop’t into the water.

The best way of making them, is to take up some of the Metall out of the pot upon the end of an Iron rod, and immediately let it drop into cool water, and there lye till it cool.

If the Metall be too hot when it drops into the water, the Glass drop certainly frosts and cracks all over, and falls to pieces in the water.

Every one that Cracks not in the water, and lies in it, till it be quite cold, is sure to be good.

The most expert Workmen, know not the just temper of the heat, that is the requisite, and therefore cannot promise before hand to make one that shall prove good, and many of them miscarry in the making, sometimes two or three or more for one that hits.

Some of them frost, but the body falls not into pieces; others break into pieces before the red heat be quite over, and with a small noise; other soon after the red heat is over, and with a great noise; some neither break nor crack, till they seem to be quite cold; other keep whole whilest they are in the water, and fly to pieces of themselves with a smart noise as soon as they are taken out of the water; some an hour after, others keep whole some days or weeks, and then break without being touched.

If one of them be snatched out of the water whilst it is red hot, the small part of the neck, and so much of the thred or string it hangs by, as has been in the water, will upon breaking fall into small parts, but not the Body, although it have as large cavities in it, as those that fly in pieces.

If one of them be cooled in the air, hanging at a thread, or on the ground, it becomes like other Glass, in all respects, as solidity, &c.

When a Glass drop falls into the water, it makes a little hissing noise, the body of it continues red a pretty while, and and [sic] there proceed from it many eruptions like sparkles, that crack, and make it leap up and move, and many bubbles do arise from it in the water, every where about it, till it cool: but if the water be ten or twelve Inches deep, these bubles diminish so in the ascending, that they vanish before they attain the superficies of the water; where nothing is to be observed, but a little thin steam.

The outside of the Glass drop is close and smooth like other Glass, but within it is spungious, and full of Cavities or Blebs.

The figure of it is roundish at the bottom for the most part, not unlike a pear pearl, it terminates in a long neck, so that never any of them are straight, and most of them are Crooked and bowed into small folds and wreaths from the beginning of the neck till it end in a small point.

Almost all those that are made in water have a little protuberance or knob a little above the largest part of the body, and most commonly placed on the side towards which the neck ends, although sometimes it be upon that side that lies uppermost in the vessel where it is made.

If a Glass drop be let to fall into water scalding hot, it will be sure to crack and break in the water either before the red heat be over, or soon after.

In Sallet Oyl they do not miscarry so frequently as in cold water.

In oyl they produce a greater number of bubbles, and larger ones, and they bubble in oyl longer than in water.

Thsoe that are made in oyl have not so many, nor so large blebs in them, as those made in water, and divers of them are smooth all over, adn want those little knobs that the others have.

Some part of the neck of those that are made in oyl, & that part of the small thread that is quenched in it cool’d, breaks like common Glass. But if the neck be broken neer the body, and the body held close in ones hand, it will crack and break all over: but flies not into so small parts, not with so smart a force and noise as those made in water, and the pieces will hold together till they be parted: and then there appears long freaks or rays upon them, pointing towards the center or middle of the body, and thwarting the little blebs or cavities of it, whereof the number is not so great, nor the size to large as in those made in water; if the Glass drops be dropt into vineger, they frost and crack, so as they are sure to fall to pieces before they be cold, the noise of falling in is more hissing than in water, but the bubbles not so remarkable.

In milk they make no noise, nor any bubbles that can be perceived, adn never miss to frost and crack, adn fall in pieces before they be cold.

In spirit of wine they bubble more than in any of the other liquors, and while they remain entire, tumble too and fro, and are more agitated than in other liquors, and never fail to crack adn fall in pieces.

By that time five or six are dropt into the spirit of wine, it will be set on flame: but receive, no particular taste from them.

In water wherein Nitre or Sal Armoniack hath been dissolved, they succeed no better than in vineger.

In oyl of Turpentine one of them broke as in the spirit of wine, but the second set it on fire, so as it could no more be used.

In Quick-silver, being forced to sink with a stick, it grew flat and rouch on the upper side: but the experiment could not be perfected, because it could not be kept under till it cool’d.

In an experiment made in a Cylindrical Glass, like a beaker filled with cold water of seven or eight onely one suceeded, the rest all cracking and breaking into pieces, onely some of the company, who taking the Glass in their hand, assoon as the drop was let fall into it, observed that at the first falling in, and for some time after, whilst the red heat lasted, red sparks were shot forth from the drops into the water, and that at the instant of the eruption of those particles, and of the bubbles which manifestly break out of it into the water, it not only cracks and sometimes with considerable noise, but the body moves and leaps, as well of those that remain whole in the water, as those that break.

A blow with a small happer, or other hard tool will not break one of the Glass Drops made in water, if it be touched no where but on the body.

Break of the tip of it, and it will fly immediately into very minuted parts with a smart force adn noise, and these parts will easily crumble into a coarse dust.

If it be broken, so that the sparks of it may have liberty to fly every way, they will disperse themselves in an orb with violence like a little Granado.

Some being rubed upon a dry tyle, fly into pieces by that time the bottom is a little flatted others not till half the rub’d off. One being rub’d till about half was ground away, and then layed aside, did a little while after fly in pieces without being touched. Another rub’d almost to the very neck on a stone with water and Emery did not fly at all.

If one of them be broken in ones hand under water, it strikes the hand more smartly, and with a more brisk noise than in the air: yeah, though it be held near the superficies none of the small parts will fly out of it but all fall down without dispersing as they do in the Air. One of them borken in Master Boyles Engine, when the Receiver is well Evacuated will fly in pieces as in the open air.

Anneal one of them in the fire, and it will become like ordinary Glass, onely the spring of it is so weakned, that it will not bend so much without breaking as before.

A Glass drop being fastned into a cement all but a part of the neck and then the tip of it broken off it made a pretty smart noise but not so great as those use to do that are broken in the hand, and though it clearly appears ot be all shiver’d within, and the colours turned grayish, the outside remained smooth, though cracked, and being taken in pieces, the parts of it rise in flakes, some Conical in shape, and so crack all over that it easily crumbled to dust.

One fastned in a ball of cement some half an Inch in thickness, upon the breaking off the tip of it, it broke the ball in pieces like a Granado.

Two or three of them sent to a Lapidary to peirce them thorow, as they do Pearls, no sooner had the tool entred into them, but they flew in pieces as the use to do when the tip of them is broken off.

Title Page For the 1662 English Translation of Antonio Neri's The Art of Glass