Already in the Middle Ages, people grinded spectacle lenses. Why did it take until 1608 before a combination of two lenses resulted in a usable telescope*? In that year, the Dutchman Hans Lipperhey showed a 'certain instrument to view the distance' to Prince Maurits in The Hague, The Netherlands.

There is not a simple answer to this question. However, it is clear that both high-quality glass and advanced grinding techniques are necessary to construct a telescope from two ordinary spectacle lenses. We know from his correspondence that Galileo himself had to struggle to find good lenses for his telescopes.

The pupil of the human eye has an opening of about 2 mm (in daylight). If we look through a spectacle glass, then we just look through about 3 mm of the lens. If we turn our eye around, we look through a different part of the glass every time. In the cases where the lens is not grinded perfectly spherical, there is no problem. Even if the focal length is not exactly the same, our eyes will correct for it. Therefore, badly grinded lenses still provide a reasonable image. The spectacle glasses that were made and sold in the Middle Ages were thus sufficient.   

However, the use of lenses in a telescope is very different from that in spectacle glasses. Here, the whole opening of the objective (front lens) is used for the image. Because the second lens, the ocular, strongly magnifies this image, the objective lens should be of excellent quality. So, we do not only need high-quality glass without gas-bubbles, but we also need to be able to grind lenses in a perfectly spherical shape. 

How did people grind lenses in the early 17^th century? Which techniques were applied? Let's have a look in the workshop of an optician of that time. Several tools for working glass lied on his workbench. He bought the glass at the glass factory in the neighbourhood. For the opticians in Middelburg, that was relatively easy. Middelburg had the oldest and maybe the best glaze company of the Netherlands within her walls. From the glass plate, the optician cut round circles with a diameter of about 3 centimetres by hand. He made a tool especially for this task (see illustration). Subsequently, he glued two little glass plates together with beeswax. The first was intended to be grinded and the other was just meant for reinforcing the first one during the grinding process. 

A mould of the desired spherical shape was carefully hammered in a copper plate. This mould was fixed on the axis of a grinding machine. Because of the ingenious construction of the machine, both the axis and mould could rotate at high velocities. The two glued pieces of glass were attached to a kind of stamper that was usually made of marble. But also iron and wood were sometimes used. This stamper pushed the two pieces of glass down into the mould, causing the glass being grinded into the desired spherical shape using additional grinding powder.    

The grinding powder was also called sand, but it consisted of quartz and feldspar. Feldspar is a very common mineral, which makes up 60 percent of earth's crust. Finally, the grinded lens was polished  using tin oxide or tripolite on a fast-rotating wooden disk that was covered with leather. Tripolite consists of the shells of certain seaweeds or algae (diatomea) and is actually the same as very fine quartz powder.  

This way of lens grinding has stayed the same for hundreds of years. The optician finally made a pair of glasses by putting the lenses in a metal or leather framework. The quality was good enough for a pair of glasses, but telescope builders had a big problem. Almost all lenses made using this method were simply too a-spherical to be used in a telescope.  

In order to make suitable lenses for telescopes, a new technique had to be invented. It seems that this technique was developed in Middelburg in 1608. Hans Lipperhey, the optician who presented his first telescope to Prince Maurits in The Hague in September of that year, had to master it. He was even able to present a binocular to the Dutch politicians on December 11, 1608. It was an instrument consisting of two parallel telescope tubes intended for use with two eyes and it provided a good image. This instrument design was approved and he received an order to build another two. In return, he received a lot of money and was even able to buy a house. It was a special achievement, because it took until 1823 to produce the first binoculars that were commercially successful. In the two-hundred years before the success of Friedrich Voigtländer from Vienna, only a handful of instrument builders had tried to construct a binocular. Just from this achievement alone, Hans Lipperhey already deserves a lot of credit. He was a real craftsman!

People who wanted to buy a telescope in 1608 or 1609, had to go to a Dutchman. In Holland, or actually Middelburg, they had the best Venetian glass and the best grinding technique. We know this, because the German astronomer Simon Marius was looking for expert German opticians to build a good telescope. His search ended eventually in Holland. It seems it was the only place where high-quality lenses could be made. Fred Watson, writer of the book 'Stargazer', writes: “Even the most professional opticians of Neurenberg – the best of Germany – could not create them.”

In November 1609, Marius had seen 'stars' next to the planet Jupiter and was very interested in a good telescope. He published 'Mundus Jovialis' ('the world of Jupiter') in which he writes about his Jupiter observations in 1614. Although Galileo is regarded as the discoverer of the moons of Jupiter, he is certainly not the first person to point a telescope toward the sky. But others before him, like Marius, published their observations later. Simon Marius is not considered to be a co-discoverer of the moons of Jupiter. He writes that he saw the moons on the same day as Galileo, but he provides the date in the Julian system. Eventually, he says that he discovered the moons in Germany and Galileo in Italy. His claim was not considered to be credible. Galileo published his observations very quickly in 1610, only ten days after his Jupiter observations. Therefore, Galileo made the history books as the discoverer of the Jovian moons. Perhaps Galileo knew that others also used the new invention to look at the sky.

What Lipperhey discovered exactly to make high-quality lenses is not entirely known. If we test medieval lenses for quality, then only the most central part of the lens passes the test. So, Lipperhey developed a new grinding technique that improved the whole lens or he discovered the diaphragm. By using the diaphragm, you only use the 'good' central part of the lens, which provides the best image. If we look at the first usable telescopes, including the ones of Galileo, they are all equipped with a diaphragm.

Hieronymus Sirturus from Milaan travelled a lot to learn everything about telescopes. In 1618, he published a book in which he complains about the grinding technique that the opticians use. The third chapter of his book 'Telescopium' is titled 'About the deterioration of the Art (of lens grinding)'. In the book, Sirturus gives a number of recommendations to grind better spherical lenses.

One of the recommendations is about making the mould more spherical. The copper mould was hammered into shape, but this caused small bumps. By removing the bumps with a file, the mould becomes smoother. He also recommended to fill little holes in the mould with liquefied lead. He also gave some hints to better assess the spherical shape.

All these recommendations improved the quality of the lenses. More and more scientists published magnificent observations. Halfway the 17^th century, there were already complete Moon atlases. The telescopes became larger and larger. Although telescopes were primarily meant for scientific purposes, a telescope in the garden or on the roof increased a persons status. That has not changed in 400 years...!

* In this article, I use the word telescope for simplicity. However, the word itself was only introduced three years after the invention. Initially, it was called 'the instrument to view the distance' or 'invention to stretch the view'.
 

Sources:
Fred Watson ‘Stargazer’
Rolf Willach, ‘The development of lens grinding and polishing techniques in the first half of the 17th century’. (Bulletin van de Scientific Instrument Society. No. 68, 2003)
Rolf Willach, ‘Der Lange Weg zur Erfindung des Fernrohres’ 2007
Internet: www.dbnl.org – Digital Library of the Dutch Literacy.