Class XXIV.] GUINAXD’S PROCESS FOR PRODUCING DISCS FREE FROM STRIDE.

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tried on ihis principle did not meet with the success whichhe had e xpectetl.

In 17 53, John Dollond directed his attention to thesame ohj-ect; and, aided by the co-operation of Euler , andof Klingenstieraa the Swedish mathematician, he suc-ceeded, after a long series of experiments, in discoveringthe due proportions of the curvatures of the two lenses,of which the object-glass of the achromatic telescope iscomposes!; the one being flint-glass (silicate of white leadand potash), the other, crown or plate (silicate of sodaand lime), which, having different refractive and disper-sive pow ers, may be so arranged, that while, by the com-bination of the two, the rays are brought to a focus, thegreater dispersion of the flint lens may be corrected bythe less dispersive power of the plate; the one beingnegative, the other positive. “ The refractive power ofthe glasis bends the rays of light falling upon, or ratherentering its surface, according to a certain law, in whichit proceeds through the medium, while the glass is of thesame kiand. The same occurs again when the ray, havingpassed t hrough the lens, arrives at the other surface andpasses iaito air, the ratio here being the inverse of theformer."’— Encyc . Met.

Hence arises the importance of avoiding any variationin the media of which the glass is composed; as theslightest difference would act as a disturbing cause to theprogress of the ray of light in its dvie course, derange therefracti'on, and distort the object.

After this principle was attained, Dollond and theFrench and German opticians experienced extreme diffi-culty ini procuring glass adapted to their purpose. TheAcademy of Sciences , at Paris , offered prizes in vain forthis object. The celebrated chemists, Macquer Roux , ofSt. Gobiain, and Auut, of Langres , devoted their attentionto it in vain; and even when procured, 3 to 3^ inches indiameter was the largest size they could obtain.

M. d 'Artigues, one of the first manufacturers of crystal(flint-gl.ass) in France , and who to his high reputation forpractical skill added that of scientific knowledge, equallyfailed, or produced insufficient results; and it remainedfor a m.an in no degree conversant with science, not aglassmaiker by trade, nor distinguished by education, butendowed with extraordinary energy, spirit of inquiry,and perseverance, to have the honour of arriving at thesolution of the difficulty.

Guinand des Brenets, near Xeufchatel, a workman inthe clock and watch trade, who had been accustomed tothe fus ing of metals in the prosecution of his business,observing that in the ordinary process the waving and thethreads., which are frequently to be seen in glass, wereremove-d by stirring and thoroughly mixing the metal bymeans <of an iron bar, applied the same process to fiiut-glass, a.nd combining it with close attention to the struc-ture, after many attempts made during his leisure hours,succeeded in producing flint-glass perfectly free fromstriae.

M. Utzschneider, of Munich , hearing the result of hisexperiments, on making further inquiry, proposed to himto join him and M. Frauenhoffer in their establishment atMunich : he accepted the offer; and one of the largestglasses resulting from their experiments, the diameter ofwhich as 9 inches, is now in the Observatory at Dorpat.

Guinaiul returned to his own country, but not being aglass-maker by profession, he prosecuted his researchesonly at intervals. He had discovered the principle; liehad earned a well-merited reputation in the world ofscience-, and promoted the researches of others; but theresults of his experiments had not attained to certaintyin practice, and he had not overcome the difficulties inthe fabrication of crown-glass, which requires the sameperfection, and the same dimensions as the correspondingflint.

In the latter years of his life, Gninand entered intocommunication with the Astronomical Society of London,and sent over some discs of flint-glass, of which Messrs.Dollon d, Hersehel, and Pearson made a favourable re-port. The largest of these glasses was 6 inches in dia-meter: and it is remarkable, that in England, which hadsupplied the Continent with flint-glass, a disc of 6 inchesshould have been regarded as a rarity.

Soon after, a Commission, composed of Messrs. Hersehel,

| Faraday, Dollond , and Koget, was instructed to pursuej the inquiry as to the manufacture of flint-glass. Mr.

I Faraday took the lead, both in his own laboratory and atj the glass-works of Messrs. Pellatt, and could not fail of| arriving at important conclusions. He changed the prin-! ciple of fabrication, and produced a borate of lead ofj remarkable purity. The Lords of the Treasury hadI found it advisable to make a relaxation of the Excise lawsin favour of the Koval Society, or persons acting forI scientific purposes under that body. But, notwithstand-ing this regulation, the interference of the officers, andthe delay in obtaining the necessary licence, proved soonerous aud inconvenient, as completely to shackle theirproceedings, and preclude all attempt to improve bymeans of experiment: and the question as to the fabri-cation of flint-glass being actively pursued in France andSwitzerland , the Commission ceased from its labours.

Shortly afterwards M. Guinaml died, without leavingany information as to his process. But in Bavaria , theworks in which he had taken part had been continuedaccording to his system; and his wife and two sons hadwitnessed his experiments, and were desirous of availingthemselves of their father’s invention for their own ad-vantage.

M. Bontemps, who had devoted much attention to themanufacture of glass generally, and particularly of suchas is required for optical purposes, was introduced byM. Lerebours, of Paris , to one of the sons of Gninand,who was endeavouring to sell his father’s secret in Eng-land, or in France . He formed an association with him,but did not succeed in arriving at any good result. Thetreaty was broken; but M. Bontemps, satisfied of thecorrectness of the principle if properly applied, continuedthe experiments at his works without excluding Guinaml;and in 1828 they succeeded in producing good flint-glass,and discs of from 12 to 14 inches, and a large quantity ofsmaller sizes. From that time the manufacture may beconsidered to have been established on a regular system.

The widow of Guinaml and her other son set up worksin Switzerland upon the father’s principles, and were suc-ceeded by M. Daguet, of Soleure , who has sent to theExhibition some of his products of moderate size, andpure in colour (14-6 et infra); but they prove the diffi-culty of producing large discs, and confirm the belief thatthere are even more impediments in fabricating crown-glass of large size, than in making good crystal. Inorder to render it free from impurity, it becomes moredifficult of manufacture, more liable to tension, and toaccidents. It requires a higher temperature. By in-creasing the facility of fusion, the disposition to attracthumidity, or to sweat, is increased. In rendering it toohard, the risk of crystallization, and imperfect vitrificationin cooling, is incurred.

The insecurity in which political events in France involved the persons engaged in industrial pursuits wasexperienced by M. Bontemps: he contended against it,however, till the year 1848; when, after attaining to ahigh degree of eminence, and receiving the decorationswhich are awarded in that country to distinguished merit,he was induced to retire from the difficulties and dissen-sions which prevailed around him, and to accept theinvitation of Messrs. Chance, Brothers, and Co., to unitewith them in the attempt to improve the quality, andextend the utility of this important branch of manufacture.They have succeeded in producing discs of extraordinarydimensions in flint of 29 inches in diameter, weighing2 cwt., and of crown-glass up to 20 inches. These discsappear to be pure in colour, good in structure, and exemptfrom those defects which tend to polarization of light,&c., and were considered by the Jury of Class X., Sir David Brewster , Sir John Hersehel, Lord Wrottesley,and others, to be so important, that they invited Messrs.Chance to submit their disc of flint to the operation ofgrinding, finishing, ami other processes, necessary inorder to ascertain the uniformity of its density through-out.*

* Since the above was written this remarkable disc hasbeen subjected to the tests required in order to prove its

2 m