With both those kinds of eye-pieces the object appears to be inverted; but eye-pieces with three lenses, by which the object is made to appear in the erect position, had been proposed by 'limits : these being found defective, Mr. Dollond endeavoured to improve upon the construction by dividing the refraction at the first and third eye-glasses between two lenses, according to the method recommended by Hitygliens ; and thus be formed eye-tubes with five lenses. But some light is always lost by reflection when it falls upon glass; and in order to diminish this evil, Dollond subsequently, retaining the Ilnyghenian construction 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 glasses), in rendering the rays of each pencil convergent after the first had dimi nished the divergency caused by the crossing at the focus of the objectglaas : he thus imocceiled in producing an eye-piece of four lenses which was nearly aplanntic, or free both from the chromatical and spherical aberrations ; and such are the telescopes now hi common use for viewing terrestrial objects.
The chief Improvements, if they may be so called, which have since been made in dloptrie telescopes, consist in the ;mans which have been adopted to remove those aberrations more completely'; and the natures of the different media which have been used for this purpoao by Dr. Blair, Sir David Brewster, and Mr. Barlow, are mentioned under TELESCOPY-.
We must, however, briefly notice the attempts that have been made of late years to improve Willa, is called optical glass. Flint glass is subject to numerous defects resulting from a want of uniform density, which defects are so common as to have obtained distinguishing names, such as stria-, or wreath, knots, threads, and Icor*. The stria, are undulating appearances in the glass, whereby the light in passing through it is refracted and dispersed in different directions, producing a wavy effect, not always apparent until the glass enters into the construction of an optical instrument. Knots are opaque particles derived from the glass pot, or particles of glass-gall, or imperfectly vitrified grains of sand. Threads and tears also consist of partially vitrified matter. The glass may also want clearness from the presence of minute bubbles or aced, as it is called, diffused through the glass in consequence of its not having been kept sufficiently fluid. It is said that good results have 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 various 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 glare adapted to their purpose. The Academy of Sciences at Parisi offered prizes in vain for unobjectionable optical glass some of the best chemists devoted their attention to the subject, but they did not succeed in obtaining larger glasses than from 3 to 3i inches in diameter. Manufacturers also made
the attempt without success. 31. Guinand, 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 stria 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 31. Frauenhoffer in their establishment at Benedietbaiiern 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. Guinand's presence greatly improved this manufactory, and achromaties 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. Merz and Mahler. Guinand, towards the close of his life, had 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 3Iessrs. Herschel, Faraday, Dollond, and Rogut, to inquire into the tnanufaeture 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 iu their regulations, as to make it very difficult to pursue the inquiry, which was, therefore, terminated. Gilliland is said to have imparted his secret to his sons before lie died, and they endeavoured to sell it on the best terms in England and France. M. Bontemps became associated with one of the eons, and iu 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 31. Daguet, of Solenre, some of whose products appeared at the Great Exhibition of 1851. Thu Jury Report, Claw XXIV.. states that these discs "confirm the belief that there are even more impedimenta in 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 iu cooling, is incurred." Soon after 1848, M. Bontempa joined the house of Messrs. Chance, Brothers, and Co., who determined to devote some of their Large means to the manufacture of optical glass. They succeeded in pro ducing discs of extraordinary dimensions—in flint of 29 inches in diameter, weighing 2 cwt., and of crown glass up to 20 inches. The large flint glass was and 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.