Museo della Specola, Bologna - Catalogue, telescopes, introduction
TELESCOPES
A Florentine manuscript of 1289 refers to some "glass lenses for spectacles recently invented, of great advantage to old people with weak vision". This confirms that the use of pieces of glass - called "vetri lenticchie" or "lenti" (lenses) on account of their lentil (lenticchie) shape - for correcting long-sightedness was known of from the end of the XIIIth century. The fact that old people’s sight could be helped by the use of such lenses, whose curvature became more accentuated as a person grew older, had important effects on society, lengthening the working life of writers, scholars and craftsmen.
It was not until the XVIth century that - against the current opinion of many scientists that observations made through transparent objects could not be confirmed - Giovanni Rucellai (1475-1525), cousin of Pope Leo X , would use a concave mirror to study the anatomy of bees, later described in the short poem Le api.
The development of the technique and the new attitude taking shape towards the study of nature led, at the end of the century, to the invention of the telescope. Given the preceding work of the spectacle-makers and the simplicity of the instrument - two lenses held one in front of the other - it is somewhat surprising that it was not invented earlier.
The lines written by Girolamo Fracastoro (Verona c. 1478-1553) in his 1538 work Homocentricorum sive de stellis liber unus, "Et per duo perspicilla ocularia, si quis perspiciat altero alteri superposito, majora multo et propinquiora videbit omnia", show fairly clearly how this idea was already in the air in the first half of the century and that not much was needed to develop it fully.
It is not clear who the first person was to build a telescope with lenses: among the names that recur are those of Zacharias Janssen and Johann Lippershey, both spectacle-makers in Middelburg, province of Zeeland. What is sure, however, is that on 2 October 1608 the General Estates of The Hague received a petition for a 30 year patent to build an instrument "for seeing things far away as if they were nearby" on the part of Johann Lippershey (Wesel 1560/70 - Middelburg 1619). The telescope in question consisted of a convex object lens and eyepiece with concave lens, had a tube about 50 cm long with a 3-4 cm diameter, and provided a three/four-fold magnification. The request however was turned down, also because other spectacle-makers had made similar claims at the same time.
The invention of the telescope can then be traced back to this date even if the Neapolitan Giambattista Della Porta (1538?-1615) claimed paternity, having written in a letter of 1586,"...to make glasses that can recognize a man several miles away" and, more explicitly, in De refratione of 1593, "Fiunt imagines ut in aere pendulae videantur, tam clare et perspicue ut nisi manibus tengas vix oculis credas". Also in the 1589 Magiae naturalis libri XX, Della Porta had spoken of a combination of concave and convex lenses for improving long-distance vision.
If the actual inventor of the telescope is not known with any degree of certainty the person who first put it to scientific use is. As is well known it was, in fact, Galileo who, having received news of the Dutch instruments, built a telescope that magnified 10 times - with a field of view of several arcminutes and a resolution power of a tenth of an arcsecond - and pointed it at the sky.
The drawback of the Galilean telescope - converging object lens and divergent eye-piece - was the reduced field of view caused by the increase in magnification. It was Kepler who, in his Dioptrice of 1611, described a system of lenses where the concave eye-piece was replaced by a convex eye-piece. A limitation was however also present in Kepler’s optical system due to the high chromatic aberration and the difficulty in working lenses with an ever greater focal length.
The leading producers of telescopes in the XVIIth and early XVIIIth centuries were Italians, among them the renowned names of Francesco Fontana (Naples c. 1585-1656), Eustachio Divini (Rome c. 1620-1685) and Giuseppe Campani (Spoleto 1636 - Rome 1715). In the competition that developed between the latter two it was Campani who came out on top, thanks especially to the help of Gian Domenico Cassini, then in Bologna, who in 1665 with telescopes by Campani discovered the red spot of Jupiter, deducing the planet’s period of rotation and accurately computing the motion of the Medicean satellites. Even after his move to Paris, Cassini went on using lenses produced for him by Campani.
The glass used in these early instruments was of very poor quality and chromatic and spherical aberration worsened the images. Despite this astronomers continued to press for ever more powerful instruments. Before the invention of mirror lenses and the perfecting of the reflecting telescope, the only way to improve the actual telescope seemed to be to increase the focal length. The result was tubes so long they threatened to bend under their own weight and supports that became increasingly unstable; tubes were built over thirty metres long, up to Hevelius’s gigantic telescope (focal length 45 metres) and the so-called "aerial telescopes" (over 60 metres), achieved by simply placing the object lens in an elevated position and collecting the image in the eyepiece, without using tubes. All at the expense of using these long instruments reasonably.
Significant improvement in the optical quality of lenses only came about in the following century when, in 1747, Leonard Euler (Basel 1707-1783) suggested using a lens made from two types of glass with different dispersive properties, designed to correct chromatic aberration. The first person to make such an achromatic lens was the English optician George Bass (c.1733-1769) - at the suggestion of Chester Moor Hall who had discovered the principle in 1733, before Euler - but it was John Dolland (London 1706-1761) and his son Peter (London 1730-1821) who began producing them in large numbers and of excellent quality [file 37 and 38].
In 1668 Newton, who had theoretically excluded the possibility of producing achromatic lenses, built the first telescope with metal mirrors, a solution which was, however, only widely accepted in the following century, finally replacing refracting telescopes in the XXth century, after which the metal mirrors were replaced by crystal ones, with aluminium or silver on the front surface, thus obtaining completely achromatic and luminous images.
