TELESCOPE. HISTORY OK THE. 



TELESCOPE, HISTORY OF TUT. 



1U 



investigating the curvature*, ha umimod that the logarithm* of tin- 

 term* expreexing the ratio of the refraction of a mean ray in paving 

 from air into gUss, and from air into water, were proportional t<> tin- 

 logarithm, of the term* expressing the ratio of the refraction* of red 

 rays in the amme media. He wa* not able to obtain from any artist a 

 1 ,u of tliu nature, in which the proposed end wai accomplished, and 

 Mr. Dollond, in a abort paper which U printed in the ' I'hiloaophical 

 Tnuuactioiu ' (1752), contorted the justness of Euler's principle on the 

 ground that it wu contrary to one which he conceived to be founded 

 on the experiment* of Newton. 



But M. Klingenstierna, a Swediah mathematician, having aoon 

 afterward*, in a Memoire which was tent to the Academic des Science*, 

 pointed out that the principle which had been adopted by Dollond wa 

 not conformable to the acknowledged law* of refraction, the Utter 

 determined immediately on having recourse to experiment. Either 

 guided by the object-gUsse* constructed under the direction of Mr. 

 Hall, or from a aerien of experiment* made by himself on the refrac- 

 tion of light in wedge* of crown and Bint glass, he discovered that by 

 employing a convex lens of the former, in combination with a concave 

 lens of the Utter kind, the rays of the different colours in each pencil 

 of light, after refraction through both, might be made to unite at the 

 focus, and thin produce an image of the object nearly free from colour. 

 For this important discovery Mr. Dollond received from the Royal 

 Society the Copleian medal. In 1765 his son, Mr. Peter Dollond, 

 diminished the aberration of light on account of the spherical form* of 

 the lenaea by combining together two convex lenses of crown glass with 

 a concave Una of flint glass between them : this construction is 

 particularly advantageous, by the increased aperture which it allow* 

 when the focal length of the compound lens U short. 



For several years after the telescopes thus improved by Dollond had 

 been in general use, Euler continued to believe that all Kinds of glass 

 differed but little from each other with respect to their dispersive 

 power, and he ascribed the success of the English artist merely to a 

 fortunate determination of the curvature of his lenses ; but having, in 

 the year 1764, received information that, by the addition of lead, glass 

 had been obtained whose diopersive power was four times as great as 

 that of the common kind, he immediately renounced his former 

 opinion ; and from that time the merit of the achromatic object- 

 glasses, as they were called, has been firmly established. The most 

 eminent mathematicians, both on the Continent and in this country, 

 have subsequently investigated, on scientific principles, the curvatures 

 which should be given to the surfaces of lenses, so that, the focal 

 length of the compound lens being assumed, the chromatical and sphe- 

 rical aberrations may be corrected. 



The arrangement of lenses for the eye-pieces of telescopes is of no 

 lea* importance than the formation of the object-glass ; and Huygheus 

 proposed (' Dioptrics,' prop, fll ), in order to diminish the refraction of 

 light at the surfaces, to substitute for the single eyeglass of the 

 common astronomical telescope two convex lenses, of such curvatures 

 that the whole refraction, or the angle between the incident and 

 emergent ray in the former construction, should be divided between 

 the two lenses. 



One mode of effecting this purpose is to place the first eye-glass, or 

 that which is nearest to the object, so as to intercept the pencils 

 coming from the object-glass before the rays are united, and thus the 

 image is formed after the refraction of the light in this lens : the 

 aecond eye-glass is then placed so that the rays falling on it, after 

 having crossed at the place of the image, are made to enter the eye 

 parallel to one another. A micrometer cannot be applied to such an 

 eye-piece, since any change in the place of the lens which is nearest to 

 toe eye would derange its adjustment : these eye-pieces can however 

 be rendered achromatic, and they have the greatest possible field of 

 view ; they have therefore been constructed for the purpose of merely 

 viewing the celestial bodies by Dollond, Ramsden, and Frauenhofer. 

 Mr. Ramsden was the first who constructed eye-pieces with two lenses 

 which were capable of being used with a micrometer : this he accom- 

 plished by placing the tube containing those lenses so that the rays in 

 the pencil*, after crossing at the focus of the object-glass, fell in a 

 diverging state upon the first eye-glass, and, after refraction in both, 

 entered the eye in parallel directions. 



With both these kinds of eye-pieces the object appears to be inverted ; 

 bat eye-pieces with three lenses, by which the object is made to appear 

 in the erect position, had been proposed by liheita : these being found 

 defective, Mr. DoUnnd endeavoured to improve upon the construction 

 by dividing the refraction at the first and third eye-glasses between 



two lease*, according to toe method recommended by Hnyghens ; and 

 thu* he formed eye-tubes with five lenses. But some light is always 

 lost by refection when it falU upon glass ; and in order to diminish 

 this evil, I>oUood subsequently, retaining the Hiiyghenian coiixtnictinn 

 in the two lenses nearest to the eye, used but one lens to perform the 

 office of the second and third (in the eye-piece with five glames), in 

 raoderinK the ray* of each pencil convergent after the first had dimi- 

 nished the divergency caused by the crossing at the focu* of the 

 object trU : be thus succeeded in producing an eye-piece of four 

 lease* which was nearly aplaxatir, or free both from the chromatical 

 and apherical aberrations ; and such are the telescopes now in common 

 nee for viewing terrestrial object*. 

 The chief improvement*, if they may be so called, which have since 



been made in dioptric telescopes, consist in the means which have been 

 adopted to remove those aberrations more completely : .mil tin- natures 

 of the different media which have been used for thia |IIII-|HMU by 

 Dr. Blair, Sir David Bruwitor, and Mr. Barlow, are mentioned un.l.V 



TCLKSCOPK. 



W<- must, however, briefly notice the attempt* that have been made 

 of Ute yean to improve wliat is called o/JiVa/ glass. Flint glass is subject 

 to numerous defect* resulting from a want of uniform density, wliii li 

 defect* are so common 0.1 to have obtained distinguishing names, such 

 asabiVr, or tcrratk, t*nti, thread*, and liart. The striic are un.iul:. tin- 

 appearances in the glass, whereby the light in passing throng 

 refracted and dispersed in different directions, producing a wavy 

 not always apparent until the glass enter* into the construction of an 

 optical instrument Knots are opaque particles derived from the glass- 

 pot, or particle* of glass gall, or imperfectly vitrified grains of sand. 

 Threads and tears also consist of partially vitrified matter. The gliuu 

 may also want clearness from the presence of minute bubbles or teed, 

 as it is called, diffused through the glass in consequence of i 

 having been kept sufficiently fluid. It is said that good results I, .-., 

 been obtained by horizontal sections of the whole contents of a pot of 

 glass. 



Our space will not allow us to do more than indicate the \.ui.n,- 

 attempts that have been made to improve optical glass. After the 

 discovery of the achromatic principle, it was seen how important, it 

 was to obtain glass of uniform density. Dollond, and the best opticians 

 abroad, had extreme difficulty in obtaining glass adapted to tln-ir 

 purpose. The Academy of Sciences at Paris offered pri/.cs in vain for 

 unobjectionable optical glass : some of the best chemists devoted tlicir 

 attention to the subject, but they did not succeed in obtaining larger 

 glasses than from 3 to 34 inches in diameter. Manufacturers also made 

 the attempt without success. M. Quinand, a watch-maker of Brenets, 

 near NeufchAtel, in Switzerland, was the first to approach the solution 

 of the difficulty. He is said to have got rid of striic by diligently 

 stirring and mixing the materials while in a state of fusion. His 

 success was such as to induce M. Utzschneider, of Munich, to join 

 him and M. Fraueuhoffer in their establishment at Benedictbauern in 

 Bavaria. He accepted the offer, and remained with them from 1805 to 

 1814; one of the largest glasses resulting from their experiments (9 

 inches in diameter) is now in the Observatory at Dorpat. liiiiii.-Mnl'- 

 presence greatly improved this manufactory, and ochromatica of 6, 7, 

 8, and 9 inches in aperture issued from the establishment, which con- 

 tinued after the death of Frauenhofer to maintain its reputation under 

 the management of Messrs. Mere and Mahler. (Juinand, towards the 

 close of his Ufe, hod some communication with the Astronomical 

 Society of London. A disc of flint glass, 6 inches in diameter, was 

 reported on favourably by Messrs. Dollond, Herschel, and Pearson. A 

 commission was also appointed, consisting of Messrs. Herschel, Faraday, 

 Dollond, and Roget, to inquire into the manufacture of flint glass. 

 Mr. Faraday pursued the inquiry for some time, and succeeded in 

 producing a borate of lead of remarkable purity. But the excise 

 officers were found to be so obstructive in their regulations, as to 

 make it very difficult to pursue the inquiry, which was, therefore, 

 terminated. Guinand is said to have imparted his secret to bis sons 

 before he died , and they endeavoured to sell it on the best terms in 

 England and France. M. Bontemps became associated with one of tin- 

 sons, and ill 1828 they succeeded in producing good flint glass, the 

 largest discs being from 12 to 14 inches. Guinand s widow and another 

 son established works in Switzerland, and were succeeded by M. 

 Daguet, of Soleure, some of whose products appeared at tin- 

 Exhibition of 1851. The Jury Report, Class XXIV., states that these 

 discs " confirm the belief that there are even more impc.liin. 

 fabricating crown glass of large size, than in making good crystal. In 

 order to render it free from impurity, it becomes more difficult of 

 manufacture, more liable to tension, and to accidents. It requires a 

 higher temperature. By increasing the facility of fusion, the disposi- 

 tion to attract humidity, or to sweat, is increased. In rendering it. too 

 hard, the risk of crystallisation and imperfect vitrification in cooling, is 

 incurred." 



Soon after 1848, M. Buuteuips joined the house of Messrs. Chance, 

 Brothers, and Co., who determined to devote some of their Urge 

 means to the manufacture of optical gloss. They succeeded in pro- 

 ducing discs of extraordinary dimensions in flint of 2! in.'],, - in 

 diameter, weighing 2 cwt., and of crown glass up to 20 inches. The 

 large flint glass was ground ami finished, and was said to be so uniform 

 in density and otherwise satisfactory, that the Jury recommended a 

 council medal to be awarded to the manufacturers. 



Mr. Ross has called attention to a defect in optical glass, which m., v 

 be detected by the searching agency of polarised light. A gloss of not 

 more than 6 inches in diameter, undergoes the annealing process with 

 dilliculty, cooling more rapidly at the surface than in the interior, and 

 n tlii tendency increase* with the si/.e, the production of a disc of 

 29 inches is justly regarded as a very remarkable work. 



Sir John Herschel, in a recent article on the telescope (' Ency. llni.'i 

 suggests that the ultimate perfection of the achromatic telescope would 

 be obtained were it possible to manufacture other species 01 

 with a much lower dispersive power than crown or plate glass, or a 

 iliUereut action on the rays of iiitcrineili.ite i, (tangibility. Tin: 

 fluoric compounds have remarkably low dispersive power, that of 



