September 13, 1900] 



NA TURE 



473 



The honour of being the originator has been claimed for three 

 men, each of whom has had his partisans. Their names are 

 Metius, Lippershey and Janssen. 



Galileo himself says that it was through hearing that some 

 one in France or Holland had made an instrument which 

 magnified distant objects that he was led to inquire how such a 

 result could be obtained. 



The first publisher of a result or discovery, supposing such 

 discovery to be honestly his own, ranks as the first inventor, 

 and there is little doubt that Galileo was the first to show the 

 world how to make a telescope (Newcomb's " Astronomy," 

 p. 108). His first telescope was made whilst on a visit to 

 \'enice, and he there exhibited a telescope viagnifyiiig three 

 times : this was in May 1609. Later telescopes which emanated 

 from the hands of Galileo magnified successively four, seven 

 and thirty times. This latter number he never exceeded. 



Greater magnifying power was not attained until Kepler 

 explained the theory and some of the advantages of a telescope 



I ide of two convex lenses in his "Catoptrics" (161 1). The first 

 rson to actually apply this to the telescope was Father 

 heiner, who describes it in his " Rosa Ursina" (1630), and Wm, 

 iscoigne was the first to appreciate practically the chief advan- 

 ces by his invention of the micrometer and application of 

 tescopic sights to instruments of precision. 

 It was, however, not until about the middle of the seventeenth 

 century that Kepler's telescope came to be nearly universal, and 

 then chiefly because its field of view exceeded that of the 

 Galilean. 



The first powerful telescopes were made by Huyghens, and 

 with one of these he discovered Titan (Saturn's brightest 

 satellite) : his telescopes magnified from forty-eight to ninety-two 

 times, were about 23 inches aperture, with focal lengths ranging 

 from 12 to 23 feet. By the aid of these he gave the first 

 explanation of Saturn's ring, which he published in 1659. 



Huyghens also states that he made object-glasses of 170 feet 

 and 210 feet focal length ; also one 300 feet long, but which 

 magnified only 600 times ; he also presented one of 123 feet to 

 the Royal Society of London. 



Auzout states that the best telescopes of Campani at Rome 

 magnified 150 times,fand were of 17 feet focal length. He him- 

 self is said to have made telescopes of from 300 to 600 feet focus, 

 but it is improbable that they were ever put to practical use. 

 Cassini discovered Saturn's fifth satellite (Rhea) in 1672, with a 

 telescope made by Campani, magnifying about 150 times, whilst 

 later, in 1684, he added the third and fourth satellites of the 

 same planet to the list of his discoveries. 



Although these telescopes were unwieldy, Bradley, with his 

 usual persistency, actually determined the diameter of Venus in 

 1722 with a telescope of 212 feet focal length. 



With such cumbersome instruments many devices were 

 invented of pointing these aerial telescopes, as they were termed, 

 to various parts of the sky. Huyghens contrived some ingenious 

 arrangements for this purpose, and also for adjusting and centre- 

 ing the eye-piece, the object-glass and eye-piece being connected 

 by a long braced rod. 



It was not, however, until DoUand's invention of the achro- 

 matic object-glass in 1757-58 that the refracting telescope was 

 materially improved, and even then the difiSculty of obtaining 

 large blocks of glass free from strire limited the telescope as 

 regards aperture, for even at the date of Airy's report we have 

 seen that 12 inches was about the maximum aperture for an 

 object-glass. 



The work of improving glass dates back to 1784, when 

 Guinand began experimenting with the manufacture of optical 

 flint glass. 



He conveyed his secrets to the firm of Fraunhofer and 

 Utzschneider, whom he joined in 1805, and during the period 

 he was there they made the 96 inches object-glass for the 

 Dorpat telescope. 



Merz and Madler, the successors of Fraunhofer, carried out 

 successfully the methods handed down to them by Guinand and 

 Fraunhofer. 



Guinand communicated his secrets to his family before his 

 death in 1823, and they entered into partnership with Bontemps. 

 The latter afterwards joined the firm of Chance Bros., of Bir- 

 mingham, and so some of Guinand's work came to England. 



At the present day MM. Feil, of Paris, who are direct de- 

 scendants of Guinand and Messrs. Chance Bros., of Birming- 

 ham, are the best known manufacturers of large discs of _ optical 

 glass. 



It is related in history that Ptolemy Euergetes had caused 

 to be erected on a lighthouse at Alexandria a piece of apparatus 

 for discovering vessels a long way off ; it has also been main- 

 tained that the instrument cited was a concave reflecting mirror, 

 and it is possible to observe with the naked eye images formed 

 by a concave mirror, and that such images are very bright. 



Also the Romans were well acquainted with the concentrating 

 power of concave mirrors, using them as burning mirrors, as 

 they were called. The first application of an eye lens to the 

 image formed by reflection from a concave mirror appears to 

 have been made by Father Zucchi, an Italian Jesuit. His work 

 was published in 1652, though it appears he employed such an 

 instrument as early as 161 6. The priority, however, of de- 

 scribing, if not making, a practical reflecting telescope belongs 

 to Gregory, who, in his "Optica Promota," 1663, discusses the 

 forms of images of objects produced by mirrors. He was well 

 aware of the failure of all attempts to perfect telescopes by using 

 lenses of various curvature, and proposed the form of reflecting 

 telescope which bears his name. 



Newton, however, was the first to construct a reflecting tele- 

 scope, and with it he could see Jupiter's .satellites, &c. En- 

 couraged by this, he made another of 6^ inches focal length, 

 which magnified thirty-eight times, and this he presented to the 

 Royal Society on the day of his election to the Society in 167 1. 



To Newton we owe also the idea of employing pitch, used in 

 the working of the surfaces. 



A third form of telescope was invented by Cassegrain in 

 1672. He substituted a small convex mirror for the concave 

 mirror in Gregory's form, and thiis rendered the telescope a 

 little shorter. 



Short also, from 1730-68, displayed uncommon ability in the 

 manufacture of reflecting telescopes, and succeeded in giving 

 true parabolic and elliptic figures to his specula, besides obtain- 

 ing a high degree of polish upon them. In Short's first tele- 

 scopes the specula were of glass, as suggested by Gregory ; but 

 it was not until after Liebig's discovery of the process of de- 

 positing a film of metallic silver upon a glass surface from a salt 

 in solution that glass specula became almost universal, and thus 

 replaced the metallic ones of earlier times. 



Shortly after the announcement of Liebig's discovery Stein- 

 heil '\Gaz. Univ. d'' Augsburg, March 24, 1856) — and later, 

 independently, Foucault {Comptes rend., vol. xliv. February 

 1857) — proposed to employ glass for the specula of telescopes, 

 and, as is well.known, this is done in all the large reflectors of 

 to-day. 



I now propose to deal with the various steps in the develop- 

 ment of the telescope, which have resulted in the three forms 

 that I take as examples of the highest development at the 

 present time. These are the Yerkes telescope at Chicago, my 

 own 5-foot reflector, and the telescope recently erected at the 

 Paris Exhibition, dealing not only with the mountings, but 

 with the principles of construction of each. When the tele- 

 scope was first used all could be seen by holding it in the hand. 

 As the magnifying power increased, some kind of support would 

 become absolutely necessary, and this would take the form of 

 the altitude and azimuth stand, and the motion of the heavenly 

 bodies would doubtless suggest the parallactic or equatorial 

 movement, by which the telescope followed the object by one 

 movement of an axis placed parallel to the pole. This did not 

 come, however, immediately. The long focus telescopes of 

 which I have spoken were sometimes used with a tube, but 

 more often the object-glass was mounted in a long cell and 

 suspended from the top of a pole, at the right height to be in a 

 line between che observer and the object to be looked at ; and it 

 was so arranged that by means of a cord it could be brought 

 into a fairly correct position. Notwithstanding the extreme 

 awkwardness of this arrangement, most excellent observations 

 were made in the seventeenth century by the users of these 

 telescopes. Then the achromatic telescope was invented and 

 mechanical mountings were used, with circles for finding posi- 

 tions, much as we have them now. I have already mentioned 

 the rivalry between the English and German forms of mount- 

 ings, and Airy's preference for the English form. The general 

 feeling amongst astronomers has, however, been largely in 

 favour of the German mounting for refractors, due, no doubt, 

 to a great extent, to the enormous advance in engineering skill. 

 We have many examples of this form of mounting. A list of 

 the principal large refracting and reflecting telescopes now 

 existing is given at the end of this paper. All the refractors in 

 this list, with the exception of the Paris telescope of 50 inches. 



NO. 161 I, VOL. 62] 



