Class X.]
OPTICAL INSTRUMENTS, TELESCOPES, Ac.
263
large or prime barrel is a solid piston, -which may bemade to rise and fall at pleasure, and is attached to apiston-rod; in the centre of the secondary barrels arealso small pistons, whose movements are simultaneous.Each rise and fall of the large piston is designed to drawoff from the receiver 53 cubic inches of air, simply by itsown elasticity. The double action, combined with therotatory motion, has been introduced with a view toeconomise both time and labour, and the absence of thevalves, to avoid the limitation of exhaustion attendantupon their use. This pump was not tried, in consequenceof no one being in attendance on the Jury to explain itsaction, &c.
Breton (France , No. 1113, pp. 12, 31) exhibits adouble-barrelled air-pump. It has, instead of valves, aglass plate sliding over apertures communicating with thereceiver and the pumps. The motion of this glass plateis produced by the mechanism which works the pump;it is very ingenious in its construction. The approximateexhaustion is first made by the ordinary alternate actionof the barrels. The system of communication is thenchanged by shifting round the glass plate, which servesas a valve during one-fourth of a revolution, when therarefied air is condensed in one barrel and sucked intothe other, whence it is ultimately ejected through a valveof oiled silk very close to the piston. On account of thedistance between the pumps and the glass plate, howeversmall the pipe of communication, the exhaustion mustbe imperfect. The syphon gauge attached to the instru-ment indicated an elastic force of only one millimetre ofa column of mercury; but a bubble of air was seen atthe top of the mercury, proving its indication to beerroneous.
Deleuil (France, No. 160, pp. 1178, 1179, 1180).This exhibitor has a double-glass barrelled air-pump, onM. Babinet’s principle, the valves being opened by meansof wires passing through the pistons. The opening ofthe valves is by this means rendered independent of theelastic force of the air remaining in the receiver. Thedegree of exhaustion which can be produced must dependoi• the air after the action of the piston. This appearedb\ the syphon gauge to be about one millimetre of mer-cury; but the top of the gauge could not be seen so asto ascertain whether any visible portion of air was there.In this pump, also, the vacuum is first approximatelymade by the alternate action of the barrels, after whichone barrel exhausts the other by suction.
Nissen (Denmark , No. 20, p. 1356) exhibits a double-acting single-barrelled air-pump, of an ingenious con-struction. This instrument only exhausted to 0'3 inch,as shown by the gauge, which was free from any visiblespeck of air. The Jury considered this pump as worthyof Honourable Mention.
Optical Instruments.
The telescope is an instrument of such high importance,that it ought to command at all times, from opticians, theincessant direction of their attention to its improvementand the bringing it to the highest possible state of per-fection. In the Exhibition, if we except those affixed toastronomical instruments, there are but few telescopes.Of these the larger are for the most part good. Wrayexhibits one with discs of a solid substance, instead ofHint glass, which deserves commendation, as a deviationfrom the beaten path, that may conduct to new and im-portant results. There are few samples in the Exhibitionof optical glass; but all are good, and give great promiseof ati increase in the use of large telescopes. Simmsexhibits several object glasses made of English glass;and Chance contributes a noble piece of apparently pureflint glass, of no less than 29 inches in diameter. Daguetsends some wonderfully pure glass, both crown and flint.Of lenses and prisms, there is not one British contributor;Fiance standing alone in the exhibition of some verybeautiful work, which reflects higli credit upon Bayerleand Bkrtaud. Of physical optics, there is but one exten-sive exhibitor, viz., Duboscq Solkii,, France (No. 1197,p. 1235), who has a beautiful collection of most delicatelyconstructed instruments, adapted for physical investiga-tion. Of microscopes there are a good many exhibited;
among which the English microscopes are found to standpre-eminent. Of lighthouses there are two, the one bein»made of glass almost colourless, and the other with thatof a greenish colour. The glass of neither is pure, therebeing many striae, &c., which must cause much light tobe scattered and consequently lost. Of spectacles andcameras we shall speak in the proper place.
Telescopes.
Yarley and Son (No. 257, p. 43G) exhibit an apparatusto be used in Gregorian telescopes, consisting of threesmall speculums, grouped together on one stem, andfitted into a telescope, under adjustment from the eveend, by means of which any one of the three mav beused at pleasure, so that the power may be changed with-out losing sight of the object. Within the tube areplaced two slides, one near the eye end, adjustable by ascrew; the other near the object end, which may bemoved to and fro. The latter carries three small spe-culums, of different foci, mounted on a steel axis, heldin a stiff frame. At the bottom of the axis is placed atoothed wheel and rack-work. This rack is kept frommoving by a long bar proceeding from the first slide, sothat it cannot move with the slide on which it lies; bythis arrangement, on moving the slide, the wheel uponit will roll against the rack, and so present the nextspeculum.
The angle at which the speculums are opposed to eachother on the block determines the number of teeth, orportions of the circle required to present each speculum.The diameter of the wheel determines the distance thatsuch portions of the wheel must traverse to put eachspeculum in true focus. The slide nearest to the eyeend is moved by a long bar, attached to it by means of ascrew, whilst its near end lies on the other slide, andover the loop-hole. The bar lias a screw handle on theoutside of the telescope, by which to pull or push thefurther slide, and also to clamp it fast to the near slidewhen in the right place. This clamping connects thetwo slides, and causes both to obey the adjusting screw.In order to determine the exact places at which to clamp,the bar is furnished with three notches, whose distancecorresponds with tlie difference of foci; a tooth snapsinto each notch as it arrives; the hand of the observerfeels this snap, and the object reappearing at the sameinstant, the screw is made fast. Having brought eachspeculum to its right distance, its perfect position iseffected w ithout trouble. The speculum wheel has threepins; agaiust one of these a notch in the bar is urged bya spring, which holds its corresponding speculum per-fectly in place, and in addition, moves the wheel andrack a little further than the hand and bar had formerlydone. This simple action separates the two hooks, andthereby detaches the apparatus from each speculumwhilst it is in use, leaving it at liberty to be governedonly by its pin, and the notch in the bar already men-tioned; the speculum by these means is held perfectly inits place. A cylindrical cap, as a protection from theweather, is made to slide over the speculums, and affordsa dark margin round the pencils of light. This con-trivance has been applied to telescopes of eight inchesfocal length, and six inches aperture.* The Gregorian form of telescope is the shortest, and consequently bestsupported on the stand; and possesses many advantages,as compared with others of equal power; from its largei proportionate aperture, it gives a smaller disc to the stars,and does not require a deep eye-piece, but it is desirableto obtain power by deeper and smaller speculums.
Varley and Son also exhibit a portable Gregorian telescope, of two inches aperture, and six inches focus.It is mourned on a brass stand, and admits of beingreadily packed away in a small box. When held againsta post er tree, the foot and telescope form a firm trian-gular bearing.
As in ordiuary Gregorians, the length of telescope in-creases the power of the small metal reflector, so thata small portion only of an object can be seen at one time