History of telescopes: Encyclopedia II - History of telescopes - Refracting telescopes

History of telescopes - Refracting telescopes

There is an archeological finding of lenses is from Gotlandia in Sweden. These so-called Visby lenses can be dated to the second half of the 11th century. 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. They have been speculated to be components from an ancient telescope. However, if it would be, maybe it was something imported from the Middle East and it was anyway forgotten until about 1600.

The earliest documented telescope was that of Roger Bacon in the 13th century. It is quite certain that prior to 1600 the telescope was unknown, except to individuals who failed to see its practical importance, and who confined its usage to curious practices or to demonstrations of "natural magic."

The practical invention of the instrument was certainly made in the Netherlands about 1608, but the credit of the original invention has been claimed on behalf of three individuals, Hans Lippershey and Zacharias Jansen, spectacle-makers in Middelburg, and James Metius of Alkmaar.

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 till 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 Catopirics (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, achieved such results.

Adapted from the Wikipedia article "Refracting telescopes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki