i86 



NA TURE 



[June 19, 1890 



In 1733, Hall had made telescopes having double object-glasses 

 on this plan, but never published the fact. Dollond, who had 

 worked independently at the subject, came to the conclusion 

 that the thing could be done, and succeeded in doing it ; the 

 invention of the achromatic telescope is with justice, therefore, 

 connected with his name. 



Although this invention was a most important one, full advant- 

 age could not be taken of it owing to the difficulty of getting 

 disks of glass large enough to make into the coTipound object- 

 glass, disks of about four inches being the largest diameter it 

 was possible to obtain. With the reflecting telescope, unham- 

 pered as it always has been by all except mechanical difficulties, 

 advance was possible, and astronomers turned to it as the only 

 means of getting larger instruments. Many most excellent 

 instruments were made on the Newtonian plan. The plan pro- 

 posed by Gregory was largely used, as in this instrument objects 

 are seen in their natural position, so that the telescope could be 

 employed for ordinary purposes. 



Many were also made on the plan proposed by Cassegrain. 

 The diagrams on the wall enable you to at once see the 

 essential points of these different forms of reflectors. 



About 1776, Herschel commenced his astronomical work ; 

 beginning with reflecting telescopes of six or seven inches, he 

 ultimately succeeded in making one of four feet aperture with 

 these instruments. As everyone knows, most brilliant discoveries 

 were effected, and the first real survey of the heavens made. 



Herschel's larger telescopes were mounted by swinging them 

 in a surrounding framed scaffolding that could itself be rotated. 

 The smaller ones were mostly mounted on the plan of the one 

 now before us, which the Council of the Royal Astronomical 

 Society have kindly allowed me to bring here. The plan nearly 

 always used by Sir William Herschel was the Newtonian, 

 though for the larger instruments he used the plan proposed 

 years before by Le Maire, but better known as the Herschelian, 

 when the observer looks diredtly at the large mirror, which is 

 slightly tilted, so that his body does not hinder the light 

 reaching the telescope. In all cases the substance used for the 

 mirrors was what is called speculum metal. 



During the present century the aperture of the refracting 

 telescope has increased enormously ; the manufacture of the 

 glass disks has been brought to a high state of perfection, par- 

 ticularly in France, where more attention is given to this manu- 

 facture than in any other country. Early in the century the 

 great difficulty was in making the disks of flint glass. M. 

 ■Guinand, a Swiss, beginning in 1784, succeeded in 1805 in getting 

 disks of glass larger and finer than had been made before, and 

 refractors grew larger and larger as the glass was made. In 

 1823 we have the Dorpat glass of 9 '6 inches, the first large 

 -equatorial mounted with clock-work ; in 1837 the 12-inch 

 Munich glass ; in 1839 the 15-inch at Harvard, and in 1847 

 another at Pulkowa ; in 1863 Cooke finished the 25-inch refractor 

 which Mr. Newall gave, shortly before his death last year, to the 

 Cambridge University. 



This telescope the University has accepted, and it is about to 

 be removed to the Observatory at Cambridge, where it will be 

 in charge of the Director, Dr. Adams. In accordance with the 

 expressed wish of the late Mr. Newall, it will be devoted to a 

 study of stellar and astronomical physics. There is every pro- 

 spect that this will be properly done, as Mr. Frank Newall, one 

 of the sons of the late Mr. Newall, has offered his personal 

 services for five years in carrying on this work. Succeeding 

 this we have the 26-inch telescope at Washington, the 26-inch 

 at the University of Virginia, the 30-inch at Pulkowa, and 

 the 36-inch lately erected at Mount Hamilton, California — 

 all these latter by Alvan Clark and his sons. By Sir Howard 

 Grubb we have many telescopes, including the 28-inch at 

 Vienna. Most of these telescopes have been produced during 

 the last twenty years, as well as quite a host of others of 

 smaller sizes, including nearly a score of telescopes of 

 about 13 inches diameter by various makers, to be employed in the 

 construction of the photographic chart of the heavens, which it 

 has been decided to do by international co-operation. 



The first of these photographic instruments was made by the 

 Brothers Henry, of the Paris Observatory, who have also made 

 many very fine object-glasses and specula, and more important 

 than all, have shown that plane mirrors of perfect flatness can 

 be made of almost any size ; the success of M. Loewy's new 

 telescope, the equatorial coiidS, being entirely due to the 

 marvellous perfection of the plane mirrors made by them. 



NO. 1077, VOL. 42] 



The reflecting telescope has quite kept pace with its elder 

 brother. 



Lassell in 1820 began the grinding of mirrors, he like Sir 

 William Herschel working through various sizes, finally com- 

 pleting one of 4 feet aperture, which was mounted equatorially. 

 Lord Kosse also took up this work in 1840 ; he made two 

 3-foot specula, and in 1845 finished what yet remains the 

 largest telescope, one of 6 feet aperture. All these were of 

 speculum metal, and all on the Newtonian form. In 1870, 

 Grubb completed for the Melbourne Observatory a telescope of 

 4 feet aperture, on the Cassegrain plan, the only large example. 

 This is of speculum metal. In 1856 it was proposed by Steinheil, 

 ' and in 1857 by Foucault, to use glass as the material for the 

 concave mirror, covering the surface with a fine deposit of 

 metallic silver in the manner that had then just been perfected. 

 In 1858, Draper, in America, completed one on this plan of 15 

 inches aperture, soon after making another of 28 inches. In France 

 , several large ones have been made, including one of 4 feet at 

 the Paris Observatory: in England this form of telescope is 

 largely used, and mirrors up to 5 feet in diameter have been 

 i made and mounted equatorially. 



' Optically the astronomical telescope, particularly the refractor, 

 : has arrived at a splendid state of excellence ; the purity of the 

 glass disks and the perfection of the surfaces is proved at once 

 by the performance of the various large telescopes. No limit has 

 yet been set to the increase of size by the impossibility of getting 

 disks of glass or working them, nor is it probable that the limit 

 will be set by either of these considerations. We must rather 

 look for our limiting conditions to the immense cost of mounting 

 large glasses, and the absorption of the glass of which the lenses 

 are made coming injuriously into play to reduce the light-gather- 

 ' ing power, though it will be probably a long time before this 

 latter evil will be much felt. 



With the reflecting telescope the greater attention given to the 

 ' working and testing of the optical surface has enabled the con- 

 cave mirror to be made with a certainty that the earlier workers 

 , never dreamed of. The examination of the surface can be made 

 ! optically at the centre of curvature of the mirror in the manner 

 j that was used by Hadley in the beginning of the last century, 

 and revived some years ago by Foucault, who brought this 

 I method of testing specula to a high degree of perfection ; in fact, 

 I with the addition of certain methods of measuring the longi- 

 ' tudinal aberrations we have now a means of readily testing 

 ' mirrors with a degree of accuracy that far exceeds the skill of 

 the worker. It enables every change that is made in the sur- 

 face during the progress of the figuring, as the parabolization of 

 the surface is called, to be watched and recorded, and the exact 

 departure of any part from the theoretical form measured and 

 corrected ; mirrors can be made of very much greater ratio of 

 aperture to focal length. I have one here where the focal length 

 is only 2\ times the aperture : such a mirror in the days of specu- 

 lum metal mirrors with the methods then in use would have 

 necessarily had a focal length of about 20 feet. The difference in 

 curvature between the centre and edge of this mirror is so great 

 that it can be easily measured by an ordinary spherometer, 

 amounting as it does with one of 6 inches diameter to 3/10,000 of 

 an inch, an amount sufficient to make the focus of the outer por- 

 tion about I inch longer than the inner when it is tested at the 

 centre of curvature. The diagram on the wall, copied roughly 

 from one of the records I keep of the progress of the work on a 

 mirror during the figuring, shows how this system of measure- 

 ments enables one to follow closely the whole operation. 



The use of silver on glass as the reflecting surface is as im- 

 portant an improvement in the astronomical telescope as the 

 invention of the achromatic telescope. It gives a permanency 

 to a good figure once obtained that did not exist with the mirrors 

 of speculum metal. To restore the surface of silver to the glass 

 speculum is only a small matter now. How readily this is done 

 may be seen by the practical illustration of the method I will 

 give. I have here two liquids — one a solution of the oxide of 

 silver, and another a reducing agent, the chief material in solu- 

 tion being sugar. I pour the two together in this vessel, the 

 surface of which has been cleaned and kept wet by distilled 

 water, which I shall partly empty, leaving the rest to mix with 

 the two solutions ; you will see in the course of about 5 minutes 

 the silver begin to form, eventually covering the whole surface 

 with a brilliant coating that can be polished on the outer surface 

 as bright as that you will see through the glass. 



Reflecting telescopes have advantages over the refracting 



