Class X.
271
low dispersive power of the zinc compounds, points to itsuse as replacing the crown glass in achromatic telescopes.Suppose it should be found practicable (and the experi-ment is recommended to the attention of artists, as one inwhich, when tried on a very small scale, some success hasactually been obtained) to form colourless and uniformglasses in which fluorine enters as a distinguished ingre-dient, in combination with silica, alumina, or othermaterials; the combination of such glass, as a convexlens, with this new material of Mr. Macs, or with ordinarycrowu glass, as a concave, might be expected to produceachromatic object-glasses of a very perfect description.The coloured dispersion to be removed, being much lessthan that of crown glass, owing to the peculiarity of thefluoric compounds, which beset the manufacture of flintglass, arising from the intense solvent power of the oxideof lead on the crucibles, and give rise to stritc and veins,would be evaded. Mr. Maes, besides two prisms of hisnew glass, of the most limpid purity and perfect freedomfrom veins or striae, has exhibited two discs of and7 inches in diameter, prepared for optical use. In an exa-mination, through faces cut on their edges, no veins orstrim were detected, and, consequently, should there ariseno objection to this material, either in point of durabilityor facility of working, it will probably prove very valu-able for the use of the optician. Some small astronomicallenses, in which zinc is substituted for crown glass, arcalso shown. Its refractive index is 3*285, and its dis-persive ratio, as compared with a flint glass of specificgravity, is as 3*55, to 0*0502.
It has been long a theorem in every treatise onoptics, that the achromatic union of the whole spectrumcannot be accomplished by any combination of dispersivemedia, and that two media of different dispersions enableus to unite but two rays precisely, the rest only approxi-mating, leaving two masses of uncemented colour, theone converging to a longer, the other to a shorter focusthan that of the united rays. A fitting combination ofthree media, admits of the exact union of three rays atdifferent points of the spectrum, and a very much greaterapproximation to union among all the rest, providedtheir scales of action on the different rays of the spectrumdiffer sufficiently. In this point of view, the new glass,whose dispersive action on the spectrum, from the intro-duction of a new metallic oxide, may very reasonably beexpected to differ from those of the crown and flintglass, will, perhaps, become available in this thirdmedium, so long a desideratum in optics: and its usemay open a new field in the theory of the constructionof telescopes. Should it prove (owing to the boracicacid it contains) less durable, or more open to atmo-spheric corrosion than the other media, its place in acompound lens will be intermediate, as a defencefrom air or moisture, and one of the conditions of thestructure may be that of a perfect coincidence of one orof both its surfaces, with those of the adjacent lenses, toallow of cementing them. A Prize Medal was awardedto M. Maes.
Dagukt (Switzerland , No. 75, p. 1271) has exhibiteddiscs of flint glass of 15J, 12?, lOf, 9?, 7^, 7, G^, and 4§inches in diameter respectively. On examination, bychords and diameters, they were all found to be good;striae were suspected to exist in the largest; but, if so, itwas in one diameter only, and but to small amount. Adiscof crown-glass, of 7 inches diameter, and one of 4|inches, were examined, and found to be good; another,of Gj inches, Mas less perfect. The specific gravity ofthe flint was stated to be nearly 4, and that of the crownto be from 3*5 to 3*G.
Mr. Daguet, by a process of his own, gives to flint-glass a degree of hardness not attained by any othermanufacturer; his glass, particularly the flint, is ingeneral use among all the best opticians, being dis-tinguished both by its homogeneousness and its peculiarproperty of resisting all decomposition by the action ofair. The Council Medal M as awarded to Mr. Daguet.
Lenses and Prisms.
Beyerle (France , No.7G5, p.1217) exhibits many lensesground with cylindrical surfaces; they are free from
spherical aberration, and give a true view on their Mholesurface: also long lenses, with the axes parallel, equallytrue throughout; they are useful in photographic regis-tration of changes in the position of magnets, &c., asadopted by Mr. Brooke.
M. Beyerle also exhibits lenses ground on curves ofdifferent foci, &c.; also many achromatic lenses of shortfocus. The M'orking of these reflects high credit uponM. Beyerld, and the Prize Medal uas awarded to him.
J amis- (France , No. 548, p. 1204) exhibits a fineassortment of prisms, chiefly crown; a large flint disc,and plane and concave reflectors, mounted on brass su ingframes, of large dimensions. Honourable Mention uasmade of M. Jamin.
Bertaud, Jun. (France , No. 1549, p. 1251) has exhi-bited some of the finest possible Nicholl's prisms; someslices of crystals, prepared to show that, in certain crys-tallized bodies, heat is unequally transmitted in differentdirections; with many other specimens of most difficult,yet beautifully cut prisms, reflecting the highest creditupon the maker. The Prize Medal uas awarded tohim.
Burox (France , No. 443, p. 1199) has exhibitedvarious prisms.
Lighthouses.
CuAxcE(CIa$sXXIV., p. 477,* Class X.) has exhibiteda design for a lighthouse, consisting of a dioptric apparatusof the first order, and constructed with revolving lensesand catadioptric zones.
This apparatus combines the principles upon u*liiehthe fixed and revolving dioptric lights of Fresnel wereconstructed, with an improved method of reflection,by which the use of metal reflectors is entirely super-seded.
In the fixed dioptric light of Fresnel, the flame isplaced in the centre of the apparatus, and within acylindric refractor of glass, of a vertical refracting power,the breadth and height of the strip of light emitted byit, being dependent upon the size of the flame and theheight of the reflector itself: above and below is placeda series of reflecting prismatic lings, or zones, for col-leetingtheupper and lower divergent rays, which, fallingupon the inner side of the zone, are refracted, passthrough the second side, uheve they suffer total re-flection, and, passing out on the outer side of the zone,are again refracted. The effect of these zones is tolengthen the vertical strip of light, the size of which isdependent upon the breadth of the flame and the heightof the apparatus.
The system adopted by Fresnel, in his revolvingdioptric lighthouse, is open to some objections. A largeflame is placed in the centre of a revolving frame, whichcarries a number of lenses, on a large scale and of variouscurvatures, for the avoidance of spherical aberration.With the view of collecting the divergent rays above theflame, an arrangement of lenses and silvered mirrors isplaced immediately over it. By this compound arrange-ment the simply revolving character of the apparatus isdestroyed, as, in addition to the revolv ing flash, a ver-tical and fixed light is at all times seen; added to whicha great loss of light must be sustained from the use ofmetallic reflectors.
The lighthouse constructed by Mr. Chance may bedescribed as Fresnel’s revolving light, rendered Iiolo-photal. It is divided into three compartments, the upperand lower of which are composed respectively of thirteenand six catadioptric zones, which produce the verticalstrip of light extending the whole length of the apparatus,and is similar to that described as Fresnel’s fixed dioptriclight. The central or catoptric compartment consists ofeight lenses of three feet focal length, each of which isthe centre of a series of eleven concentric prismaticrings, designed to produce the same refractive effect as asolid lens of equal size. These compound lenses aremounted upon a revolving frame, and transmit horizontalflashes of light as they successively rotate. The motion iscommunicated to the frame by a clock movement, andperforms one revolution in four minutes; consequently,as there are eight lenses, a flash of light is transmitted