astro14

The technology for producing lenses has been known from before recorded history. Early lenses were not made from glass, but carved and ground from rock crystal (quartz). It is difficult to determine if artifacts found by archeologists are jewelry or deliberate attempts at producing lenses. An overview can be found in G. Sines & Y. Sakellarakis:"Lenses in Antiquity", American Journal of Archaeology 91 (1987), 191-196.

The recorded use of lenses appears in Greek and Roman sources (see Lens (optics)).

The beginnings of telescopic astronomy can be traced back to the Iraqi astronomer Ibn al-Haytham (known as Alhazen in the West), the "father of optics", in the 11th century. His work was influential in the development of the modern telescope.^[1]

There is an archaeological finding of lenses from Visby on the island of Gotland in Sweden. These Visby lenses can be dated to the second half of the 11th century. The form of the lenses is such that the one half is near a perfect ellipsoid and the other flat, making a perfect tool for handling light beams. Some of these lenses have a silver mounting and have been used as pendants. There are also unmounted lenses that may have been used as a loupe. These lenses have been speculated to be components from an ancient telescope. [2][3]

From approximately the 11th century in Europe, 'reading stones' - magnifying lenses placed on the reading material - are well documented, as well as the use of lenses as burning glasses. Robert Grosseteste wrote several scientific treatises between 1230 and 1235 including De Iride (Concerning the Rainbow) in which he said:

This part of optics, when well understood, shows us how we may make things a very long distance off appear as if placed very close, and large near things appear very small, and how we may make small things placed at a distance appear any size we want, so that it may be possible for us to read the smallest letters at incredible distances...

Roger Bacon was a pupil of Grosseteste at Oxford, and is frequently stated as having described a telescope in the 13th century, however it is not certain if he built a working model.

It is generally considered that in Europe spectacles for correcting long sightedness with convex lenses were invented in Northern Italy in the late 13th to early 14th century. It is possible they had been invented and were in use in China before this period, but the knowledge had not spread, and the invention in Italy was independent. The invention of the use of concave lenses to correct near-sightedness is ascribed to Nicholas of Cusa in 1451.

Thus, from the middle of the 15th century onwards, the availability of lenses for spectacles means that it was possible for many individuals to discover the principles of a telescope using one concave and one convex lens, but there is little clear documentation and no physical evidence found of such a discovery.

There is strong documentary evidence, but no physical evidence, that both reflecting and refracting telescopes were known in England in the late 16th century. Writings by John Dee and Thomas Digges in 1570 and 1571 respectively ascribe the use of both reflecting and refracting telescopes to Thomas' father, Leonard Digges. This is independently confirmed by a report by William Bourne in approximately 1580. However, this knowledge was not exploited, and it was not until the early 17th century in the Netherlands that the knowledge of construction and use of telescopes became widespread.

The practical exploitation of the instrument was certainly achieved in the Netherlands about 1608, but the credit of the original invention has been claimed on behalf of three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar also known as Jacob Adriaanszoon.

The original Dutch telescopes were composed of a convex and a concave lens, and telescopes so constructed do not invert the image. Telescopes seem to have been made in the Netherlands in considerable numbers soon after the date of their invention, and rapidly found their way all over Europe.

Galileo, happening to be in Venice in about the month of May 1609, heard that a Belgian had invented a perspective instrument by means of which distant objects appeared nearer and larger, and that he discovered its construction by considering the effects of refraction. Galileo states that he solved the problem of the construction of a telescope the first night after his return to Padua from Venice, and made his first telescope the next day by fitting a convex lens in one extremity of a leaden tube and a concave lens in the other one. A few days afterwards, having succeeded in making a better telescope than the first, he took it to Venice, where he communicated the details of his invention to the public, and presented the instrument itself to the doge Leonardo Donato, sitting in full council. The senate, in return, settled him for life in his lectureship at Padua and doubled his salary. Galileo may thus claim to have invented the telescope independently, but not until he had heard that others had done so.

Galileo devoted his time to improving and perfecting the telescope, and soon succeeded in producing telescopes of greatly increased power. His first telescope magnified three diameters; but he soon made instruments which magnified eight diameters, and finally one that magnified thirty-three diameters. With this last instrument he discovered in 1610 the satellites of Jupiter, and soon, afterwards the spots on the sun, the phases of Venus, and the hills and valleys on the Moon. He demonstrated the revolution of the satellites of Jupiter around the planet, and gave rough predictions of their configurations, proved the rotation of the Sun on its axis, established the general truth of the Copernican system as compared with that of Ptolemy, and fairly routed the fanciful dogmas of the philosophers. These brilliant achievements, together with the immense improvement of the instrument under the hands of Galileo, overshadowed in a great degree the credit due to the original inventor, and led to the universal adoption of the name of the Galilean telescope for the form of the instrument invented by Lippershey.

Johannes Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611). The first person who actually constructed a telescope of this form was the Jesuit Christoph Scheiner, who gives a description of it in his Rosa Ursina (1630).

William Gascoigne was the first who practically appreciated the chief advantages of the form of telescope suggested by Kepler, viz., the visibility of the image of a distant object simultaneously with that of a small material object placed in the common focus of the two lenses. This led to his invention of the micrometer and his application of telescopic sights to astronomical instruments of precision. But it was not till about the middle of the 17th century that Kepler's telescope came into general use, and then, not so much because of the advantages pointed out by Gascoigne, but because its field of view was much larger than in the Galilean telescope.

The first powerful telescopes of this construction were made by Christiaan Huygens, after much labour, in which he was assisted by his brother. With one of these, of 12-ft. focal length, he discovered the brightest of Saturn's satellites (Titan) in 1655, and in 1659 he published his Systema Saturnium, in which was given for the first time a true explanation of Saturn's ring, founded on observations made with the same instrument. The sharpness of image in Kepler's telescope is very inferior to that of the Galilean instrument, so that when a high magnifying power is required it becomes essential to increase the focal length.

Giovanni Cassini discovered Saturn's fifth satellite (Rhea) In 1672 with a telescope of 35 ft., and the third and fourth satellites in 1684 with telescopes made by Campani of 100- and 136-foot focal length. Christian Huygens states that he and his brother made object-glasses of 170 and 210 ft. focal length, and he presented one of 123 feet to the Royal Society of London. Adrien Auzout (died in 1691) and others are said to have made telescopes of from 300 to 600 ft. locus, but it does not appear that they were ever able to use them in practical observations. James Bradley, on December 27, 1722, actually measured the diameter of Venus with a telescope whose object glass had a focal length of 212 ft. In these very long telescopes no tube was employed, and they were consequently termed aerial telescopes. Huygens contrived some ingenious arrangements for directing such telescopes towards any object visible in the heavens-the focal adjustment and centring of the eyepiece being preserved by a braced rod connecting the object glass and eyepiece. Other contrivances for the same purpose are described by Philippe de la Hire (Mém. de l'Acad., 1715) and by Nicolaus Hartsoeker (Miscel. Berol., 1710, vol. i. p. 261). Telescopes of such great length were naturally difficult to use, and must have taxed to the utmost the skill and patience of the observers. One cannot but pay a passing tribute of admiration to the men who, with such troublesome tools, ach