[Class X.
every thirty seconds to the horizon. The system of afixed light, varied by flashes from prisms, originated jwith Fresnel.
The apparatus is lighted by an Argand lamp, withfour concentric wicks: it is provided with four sizes ofburners, the largest of which is inches, and thesmallest T6 inch in diameter, and is stated to consume1 pound 10 ounces of oil per hour; carcel oil beingemployed.
The interior edges of the framework, carrying thecatadioptric zones, are silvered; the workmanship is notcharacterized by any great degree of finish; a fact in itsfavour, as any great degree of finish, or adoption ofornament, would involve an increased outlay of capitalwithout compensating advantages.
'Flic glass of which the apparatus is manufactured isdark and of a greenish colour; this we are informed isdue to its having been subjected, in melting, to a veryhigh temperature, with a view of draining off the alkali,which, by rendering it dryer and harder, would prevent theexuding of moisture from the surface, a defect to whichglass is liable when exposed to the air; it is, however,probable that the colour will have no injurious effect, asit is known that such tinge does not affect the trans-mission of light. The presence of stria?, which areobservable in many points, will, by scattering the rays,cause a loss of light to some amount.
Wilkins (Class VII., No. 157, pp. 320—328) hasexhibited a revolving catadioptric apparatus for a light-house of the first class.
This, also, is constructed upon the holophotal system,and does not differ very materially from that exhibitedby Chance. For the moveable cylindrical lenses, is sub-stituted a single revolving cylinder composed of fourlenses, alternated with an equal number of fixed ones,according to the succession of flashes to be producedduring each revolution. A new arrangement is alsointroduced, whereby the friction rollers which revolveon two parallel planes, may be so fitted on an ironaxis with regulating screws, that, when one part of theplate becomes worn, they may be adjusted to anotherpart. The glass, though white in colour, is not morefree from stria? than that exhibited by Mr. Chance,and though great finish and ornament are discernible inthe manufacture of the apparatus, these would seem tobe rather objectionable than otherwise. For a detaileddescription, see Illustrated Catalogue, Class VII.
The Jury make Honourable Mention of this light-house.
Dubosq-Soleil (France, No. 1197, p. 1235) has exhi-bited Silbermau’s heliostat. The principle of this instru-ment is quite different from that of S’gravesandes,which is that of making the perpendiculars to thereflecting plane, describe an oblique cone. In theinstruments exhibited, the reflector, by a peculiar gimbalsuspension, is always kept equally inclined to two axes,one of which is fixed, and directed towards the linewhich the reflected ray is to take; the other is kept bya clock motion always parallel to the sun’s direction, theplane of the reflected ray being by the same gimbalsuspension, kept perpendicular to the plane in whichboth these axes lie; thus the incident ray parallel to theone will be reflected parallel to the other.*
Dubosq-Soleil (No. 1197, p. 1235) has also exhibiteda saccharometer. A double gauge is formed by a rotato-polarizing plate of quartz and Iceland spar, of which twosemicircles, juxtaposed, and viewed through a telescopeof about double magnifying power, are brought to exactcoincidence of colour, by turning the spar. In thissituation, the images are viewed through au empty tube,closed at either end with glass, and of a given length.The saccharine solution is passed in, and the quality ofcolour disturbed: the semicircles no longer match: theyare then re-adjusted by sliding one upon the other twoachromatized prisms of Iceland spar acting against eachother, so as to leave their difference of thickness out-
* See Comptes Iiendus de 1’Academic,” 1S43, tomexvii., p. 1319.
standing, the amount being estimated in parts of a finelydivided scale, which is graduated so as to show, by in-spection, the saccharine matter present. This is abeautiful and delicate instrument, carefully executed andwell-finished.
Dubosq-Soleil (No. 1197, p. 1285) has also exhi-bited Bravais’ haloscope, for the exhibition of all thephenomena* connected with halos, parhelions, See. Thisbeautiful instrument consists of a clock-movement forthe purpose of giving a rapid revolution to a verticalaxis; two glass prisms (one hollow'for the reception ofwater); a quadrangular prism, and a small arm carryinga mirror, all adapted for mounting on the vertical axis;two opaque plates of glass for the purpose of obscuringone or two sides of the prisms, as necessary, and a plateof glass mounted. To produce a representation of theparhelion, it is necessary to place a candle in a darkenedchamber, on a level with, and 10 or 12 feet distantfrom, the glass prism on its axis of rotation, the prismhaving two sides covered. On looking in a horizontal; direction, the parhelion circle will become visible. Byobscuring one side of the prism, only three horizontallines are seen; two being formed from the exterior andone from interior reflection.
To produce an imitation of the white parhelion, thecandle is placed in the same position, and two sides ofthe prism are again darkened; when, on looking at theparhelion circle, at a distance of 120° from the candle,the white one will be perceived.
Coloured parhelions maybe exhibited by means of theprism of water, viewed at an angle of 20° or 25 . Oneside of the prism being obscured, the coloured parhelion,
I red on the side near the caudle, with a white train pro*
I longed to 15° or 20", will be seen,j To imitate coloured parhelions, formed at a distanceI of 98 3 from the sun, the arrangements are the same. Byj obscuring the prism, and looking at the distance of 98 Jfrom the candle, a coloured spot will be observed, red on| the side opposite to the candle, and less bright than theparhelion.
The circmnzenithal tangent of the arc maybe imitatedby means of the prism of water, the candle being loweredfrom its original position, and two sides of the prismdarkened; on placing the eye in a proper position, the1 image will then appear reflected upon the ceiling.Several changes may be produced by simply turning theprism. The eireumzenithal arc may be produced by theabove arrangement, the position of the eye only beingaltered.
To imitate the anthelion, it is necessary to make useof the quadrangular prism; its large transparent surfaceis turned towards the observer, and the candle placed alittle above the prism, at the distance of six or eight feetfrom it. A small circular mirror is then so placed thatits centre is on a level with the top of the prism. Theeye, being situated behiud the prism, will perceive aluminous circle described beneath its edge. If a little slipof paper be placed beneath, it will be perceived moredistinctly without moving. By moving the position ofthe eye, several distinct phenomena will become visible.There are means of adjusting the apparatus, to render theimage for a time permanently fixed. Other phenomenabesides those detailed can be shown by this instrument,and great merit is due to M. Bravais, the inventor.
Dubosq-Soleil, besides these, exhibits a very greatvariety of delicately constructed philosophical instru-ments, for exhibiting the phenomena of polarized lightand other physical experiments: such as a eyano-polarimeter, for observing the polarization of the sky ;Arago’s apparatus for exhibiting the interference ofpolarized light: Fresnel’s screw for polarizing glass bycompression; Babinet’s goniometer, Brewster’s stereo-scope, &c.; M. Dubosq-Soleil also exhibits an apparatus,invented by himself, for fixing the charcoal points forelectric light.
A Council Medal was awarded to M. Dubosq-Soleil.
Spectacles and Opera Glasses.
It is with regret we observe that exhibitors of spectaclesin the British portion of the Exhibition have done nothing