---4,' ,..' Fm. 14. - Herschelian reflector.

Herschel to avoid the loss of light from reflexion in the small mirror of the Newtonian telescope. It has several disadvantages. (1) The upper part of the observer's head must necessarily obstruct some of the rays which would otherwise fall on the large mirror ; but when a telescope of very large aperture is employed the loss of light thus occasioned is comparatively insignificant. Moreover, disturbance of the air in front of the telescope is created by heat from the observer's head and body, and this is fatal to the best definition. To avoid the latter drawback Sir John Herschel (Ency. Brit., 8th ed., art. "Telescope," vol. xxi. p. 128) suggested the employment of a small right-angled prism of total reflexion placed close to the eye-lens of the eye-piece, to permit the observer to view the image by looking in a direction at right angles to the eye-piece, and therefore at right angles to the tube. (2) Iii consequence of the tilting of the mirror aberration is created, and this increases rapidly with increased tilting. The construction is thus limited to telescopes in which the proportion of aperture to focal length is not too great. In Herschel's 40-feet telescope the proportion was 1 to 10, and the construction would hardly be applicable to modern telescopes, in which the proportion often rises to 1 to 5 or B. Yet, when exceedingly faint objects have to be observed, this form of telescope has great advantages. Herschel found that some objects which he discovered with such an instrument could not even be seen when the same telescope was used in the Newtonian form. The front view telescope, however, has hardly been at all employed except by the Herschels. But at the same time none but the-Herschels have swept the whole sky for the discovery of faint nebula; ; and probably no other astronomers have worked for so many hours on end for so many nights as they did, and they emphasize the easy position of the observer in using this form of instrument.

Construction of Object-Classes.

The first point is the selection of glass disks of suitable quality. Testing The requisites are (1) general transparency and freedom from object-mechanical defects, such as specks, air-bubbles, &c.; (2) homogeneity; glasses. (3) freedom from internal strain. The disk being roughly polished on the sides, faults of the first class are easily detected by inspection. In order to secure the maximum of light grasp for aperture it is desirable that the lass should be as colourless as possible; if the roughly polished disk is laid upon white paper the amount of discoloration can be readily estimated by comparing the colour of the sheet as seen directly with that seen through the glass. Fraunhofer's glass was far from colourless, Dollond's more coloured still ; and we have shown that, for purposes when extreme light grasp is not an object,-the less transparency of such glass to the blue rays of the spectrum affords advantages for a better correction of the chromatic aberration of rays in the brighter part of the spectrum. The amount of light excluded by specks, air-bubbles, or even scratches is quite insignificant ; but these blemishes create diffraction phenomena and scattered light in the field, which are very injurious to the performance of the instrument, especially when faint objects are searched for in the neighbourhood of brighter ones. It is essential for a telescope lens that the glass should be perfectly homogeneous ; that is, the refractive index must be identical for every part of the disk. This can be tested with extreme delicacy by grinding the disk into the form of a lens and testing it by Toppler's method,' described under Orrice (vol. xvii. p. 805). If the disk is intended for a concave lens and is already so thin that it becomes undesirable to make it thinner at the edges by converting it, in the first place, into a convex lens, it may be tested by placing one of its surfaces in contact with and at right angles to' the axis of a crown lens of known perfection, and testing the combination by Toppler's method. If a glass disk is not properly Anneal= annealed - that is, if it has been too oprickly cooled, so that the ing. outer shell has hardened before the inner portion - the finally solidified mass must be in a state of tension, like that of "Rupert's drops." Unless cooled very gradually an optical disk would fly to pieces, but a very much smaller defect in the annealing process would be fatal for refined optical purposes. Changes of temperature would produce changes of curvature, and the lens would also change its form when successive portions of the strained outer shell were removed in the process of grinding and polishing. Fortunately defects in annealing are very easily detected by means of the polariscope. The polished disk is placed in light reflected from a polarizing surface, such as a sheet of glass blackened at the back, and examined with a Nicol's prism as an analyser. If the bright rings and black cross (see LIGHT, vol. xiv. p. 613) are visible the disk is unfit for use ; but, since few disks are so perfectly annealed as not to show a trace of the black cross, such as show it in no marked degree may be safely employed. Perfect annealing has now become the most difficult portion of the art of making optical glass, and large disks (more particularly of crown glass) are rejected. by the optician more frequently for defects in annealing than for any other cause.

The disks having been selected, their refractive and dispersive powers determined, and the radii of curvature computed, it remains to convert the disks into lenses with surfaces of the required curvature, and to complete the object-glass. The work consists of five distinct operations - (1) rough grinding by a revolving tool supplied with sand and water ; (2) fine grinding with emery ; (3) polishing with oxide of iron, rouge, or putty powder, the grinder being faced with fine cloth, satin, paper, or - best of all - pitch ; (4) centring ; (5) figuring and testing. These processes are essentially of a technical character, and can only be familiar to those who practise the art. The details would be out of place here, but are well described in a lecture delivered by Sir Howard Grubb at the Royal Institution, 6th April 1886, and printed in Nature, 27th May 1886.

Construction of Specula.

Con- The composition of metallic specula in the present day differs struction very little from that used by Sir Isaac Newton. Many different of alloys have been suggested, some including silver, nickel, zinc, or specula. arsenic ; but that which has practically been found best is an alloy of four equivalents of copper to one of tin, or the following proportions by weight: - copper 252, tin 117.8. Such speculum metal is exceedingly hard and brittle, takes a fine white polish, and when protected from damp has little liability to tarnish. The process of casting and annealing, in the case of the specula of the great Melbourne telescope, was admirably described by Dr Robinson in Phil. Trans., 1869, vol. clix. p. 135. Shaping, polishing, and figuring of specula are accomplished by methods and tools precisely similar to those employed in the construction of lenses. The reflecting surface is first ground to a spherical form, the parabolic ;glue being given in the final process by regulating the size of the pitch squares and the stroke of the polishing machine. The process of testing is identical with that of an object-glass.

' Soon after Liebig's discovery of a process for depositing a film of pure metallic silver upon glass from a salt of silver in solution, Steinheil (Getz. Univ. d' Augsburg, 24th March 1856), and later, independently, Foucault (Comptes Rendus,vol. xliv., February 1857), proposed to employ glass for the speeds of telescopes, the reflecting surface of the glass speculum to be covered with silver by Liebig's process. These silver-on-glass specula arc now the rivals of the achromatic 'telescope, and it is not probable that many telescopes with metal specula will be made in the future. The best speculum metal and the greatest care are no guarantee of freedom from tarnish, and, if such a mirror is much exposed, as it must be in the hands of an active observer, frequent repolisbing will be necessary. This involves refiguring, which is the most delicate and costly process of all. Every time, therefore, that a speculum is repolished, the future quality of the instrument is at stake ; its focal length will probably be altered, and thus the value of the constants of the micrometer also have to be redetermined. Partly for these reasons the reflecting telescope with metallic mirror has never been a favourite with the professional astronomer, and has found little employment out of England. In England, in the hands of the Herschels, Rosse, Lassell, and De la Rue it has done splendid service, but in all these cases the astronomer and the instrument-maker were one. The silver-on-glass mirror has the enormous advantage that it can be resilvered with little trouble, at small expense, and without danger of changing the figure. Its chief work has been done in the hands of Draper and Common, who were the engineers, if not the actual constructors, of their own instruments. Glass is lighter, stiffer, less costly, and easier to work than speculum metal. The silvered mirrors have also some advantage in light grasp over those of speculum metal, though, aperture for aperture, the former are inferior to the modern object-glass. Comparisons of light grasp derived from small, fresh, carefully silvered surfaces are sometimes given which lead to illusory results, and from such experiments Foucault claimed superiority for the silvered speculum over the object-glass. But the present writer has found from experience and careful comparison that a silvered mirror of 12-inches aperture mounted as a Newtonian telescope (with a silvered plane for the small mirror), when the surfaces are in fair average condition, is equal in light grasp to a first-rate refractor of 10-inches aperture, or area for area as 2 : 3. This ratio will become more equal for larger sizes on account of the additional thickness of larger object-glasses and the consequent additional absorption of light in transmission.

Mounting of Telescopes.

The proper mounting of a telescope is hardly of less importance Mount-than its optical perfection. Freedom from tremor, ease and deli- iug of eacy of movement, facility of directing the instrument to any teledesired point in the heavens, are the primary qualifications. Our scopes. limits forbid an historical account of the earlier endeavours to fulfil these ends by means of motions in altitude and azimuth, nor can we do more than refer to mountings such as those employed by the Herschels, or those designed by Lord Rosse to overcome the engineering difficulties of mounting his huge telescope of 6 feet aperture. Both are abundantly illustrated in most popular works on astronomy, and it seems sufficient to refer the reader to the original descriptions.' We pass, therefore, directly to the equatorial telescope, the instru- Equater2 went par excellence of the modern extra-meridian astronomer, and ial. relegate to the article TRANSIT CIRCLE (q.v.) a description of those mountings in which the telescope is simply a refined substitute for the sights or pinnies of the old astronomers. The equatorial in its simplest form consists of an axis parallel to the earth's axis, called the "polar axis" ; a second axis, at right angles to this, called the " declination axis" ; and a telescope fixed at right angles to the latter. In fig. 15 AA is the polar axis ; the telescope is attached to the end of the declination axis ; the latter " rotates in bearings attached v to the polar axis, and concoaled by the telescope itself.

The telescope is counterpoised by a weight '7?

attached to the opposite end of the declination axis.

The lower pivot of the polar axis rests on a cup bearing at no. 15. - Equatorial telescope.

C, the upper pivot upon a strong metal casting 31M, attached to a stone pier S. A vertical plane passing through AA is therefore in the meridian, and, when the declination axis is horizontal, the telescope moves in the plane of the meridian by rotation on the declination axis only. Thus, if a graduated circle BB is attached to the declination axis, together with the necessary microscopes or verniers V, V for reading it (see TRANSIT CIRCLE), so arranged that when the telescope is turned on the declination axis till it is parallel to AA the vernier reads 0° or 90°, and when at right angles to AA 90° or 0°, then we can employ the readings of this circle to measure the polar distance or declination of any star seen in the telescope, and these readings will also be true (apart from the effects of atmospheric refraction) if we rotate the instrument through any angle on the axis AA. Thus one important attribute of an equatorially mounted telescope is that, if it is directed to any fixed star, it will follow the diurnal motion of that star from rising to setting by rotation of the polar axis only. If we further attach to the polar axis a graduated circle DD, called the "hour circle," of which the microscope or vernier R reads Oh when the declination axis is horizontal, we can obviously read off the hour angle from the meridian of any star to which the telescope may be directed at the instant of observation. If the local sidereal time of the observation is known, the right ascension of the star becomes known by adding the observed hour angle to the sidereal time if the star is west of the meridian, or subtracting it if east of the meridian. Since the equatorial is unsuitable for such observations when great accuracy is required (see TRANSIT CIRCLE), the declination and hour circles of an equatorial are employed not for determination of the right ascensions and declinations of celestial objects, but for directing the telescope with ease and certainty to any object situated in a known position, and which may or may not he visible to the unaided eye, or to define approximately the position of an unknown object. Further, by causing the hour circle, and with it the polar axis, to rotate by clockwork or some other mechanical contrivance at the same angular velocity as the earth on its axis, but in the opposite direction, the telescope will automatically follow a star from rising to setting.

Equatorial mountings may be divided into five types. (A) The Types o pivots or bearings of the polar axis are placed at its extremities. equator-The declination axis rests on bearings attached to opposite sides of ials.

the polar axis. The teleScope is attached to one end of the declination axis, and counterpoised by a weight at the other end, as in fig. 15. (B) The polar axis is supported as in type A ; the telescope is placed between the bearings of the declination axis and is mounted symmetrically with respect to the polar axis ; no counterpoise is therefore requisite. (C) The declination axis is mounted on the prolongation of the upper pivot of the polar axis ; the telescope is placed at one end of the declination axis and counterpoised by a weight at the other end. (D) The declination axis I Herschel, Phil. Trans., 1795, vol. lxxxv. p. 347 ; Rosse, Phil. Trans., ISO, p. 503, and 1861, p. 681.

The eye-piece of the telescope is placed in the upper pivot of the polar axis ; a portion or the whole of the axis of the telescope tube coincides with the polar axis. Mountings of types A and B - that is, with a long polar axis supported at both ends - are often called the " English mounting," and types C and D, in which the declination axis is placed on the extension of the upper pivot of the polar axis, are called the "German mounting," from the first employment of type C by Fraunhofer. A description of some of the best examples of each type will illustrate their relative advan- tagPs or peculiarities.

type A. Fig. 15 may be taken as a practical example of the earlier equatorials as made by Troughton in England and afterwards by Garnbey for various Continental observatories. In the Phil. Trans. for 1824 (part 3, pp. 1-412) will be found a description by Sir John Herschel and Sir James South of the equatorial telescope which they employed in their measurements of double stars. The polar axis was similar in shape to that of fig. 15 and was composed of sheets of tinned iron. In Smyth's celebrated Bedford telescope the polar axis was of mahogany. Probably the best example of this type of st ,\ .

mounting applied to a refractor is that made by the elder Cooke of York for Mr Fletcher of Tarnbank ; the polar axis is of cast iron and the mounting very satisfactory and convenient, but instrument on a small scale, and fig.

k w e& scale, the upper part of the tube ,, and polar axis being omitted. The 4, • NINIZIM figures show the telescope directed i',, 'MC* Alga to the pole, the hour circle being ..f sammessuPtamnswasin WatliNAitiiiiktm .ilatraiiieliklit441114.11114 Will attAlu6111 set 6h from the meridian. The :. 11111"11.1, -: -, ....1.1 11,--, '' Fm. 17. - Section of Melbourne reflector.

polar axis consists of a hollow Fro, 16. - Melbourne reflector.

cone C (fig 17) of cast iron hearings is relieved by a weight. The friction of the upper pivot is re.

axis. This pivot a is terminated by a piece of chilled cast iron polished flat on its lower face, which face reT circular rim is accurately toothed to fit a square threaded endless screw E, which is turned by the driving clock.

volves in contact with a piece of bell metal, flat on its upper and partly spherical on its lower side, bearing in A toothed wheel attached to H and acted on by a pinion a correspondingly shaped annulus, formed to receive it connected with a hand-wheel affords an easy means or in the cast-iron block which is attached to the setting the instrument in hour angle, or pier. This arrangement enables the bell-metal moving the telescope quickly in right ascencushion to take its own position when the direcsion. The telescope is clamped by iron bands tion of the polar axis is slightly changed in process of adjustment. The pressure of the pivot on with and forms one extremity of the deelina.. - L21.11r il'VZINIAMMIL74-4)-117P"ZI tion axis. The counterpoise U is attached to Tthe whole of a night's work ; thus the observer, the other extremity. There is an elegant ar- i in order to direct the instrument on a partirangement for diminishing the friction of the calm. object, has only to set an index connected declination axis, which our limits do not per- with the polar axis to the star's right ascension nit us to describe, and the means for clamping upon the hour circle, without the trouble of computing and giving slow motion in declination do not the hour angle at the instant of observation. This require special notice. The reader is referred arrangement was first introduced by Airy.

for a fuller description to Phil. Trans., 1869, • .1" The whole mounting is very massive, but very incon• ----411" ''' - ' • pp. 127-161. The telescope is of the Cassegrain ,,_tivrAFf "...--1 venient to use when a great many different objects form, the mirror having a 4-fect aperture and \ .," N- -7 - / have to be examined on the same night ; but on ac34-feet focal length. 1• i count of its freedom from tremor and the excellencedrivingrpe B. The best existino. examples of type B are sir "•• ./ .._, Airy's equatorial at Greenwich, the equatorial longed study of a single object or for long photoat Liverpool (also designed by Airy), and the Flo. 18. - Greenwich equatorial.

graphic exposures? Quite recently Sir Howard Grubb New photographic equatorial recently erected at the Paris observatory. has signed a contract to make a telescope of 28-inches aperture and Green28-feet focal length,2 which is to be substituted for the present tele- wich scope by )Herz & Son of 124-inches aperture and 18-feet focus. Fig. telescope. 19 is engraved from a photograph of the model of the original polar axis. The mode] was prepared to illustrate the manner in which the new telescope is to be mounted, and we are indebted for the picture unfortunately no detailed description has been published. In recent years no noteworthy refractors have been mounted on this plan ; but type A reat has been chosen by Grubb for the great 3lelel- bourne reflector, with marked ingenuity of ,,gilf:r.7urne adaptation to the peculiar requirements ,..escope. of the case. Fig. 16 shows the whole i See the detailed account in Greenwich Observations, 1568.

Vienna telescope (Grubb) 27-inches aperture, focal length 15'5 apertures Washington „ (Clark) 26 „ 15.0 „ Pulkowa „ (Clark) 20 SI 18'0 „ -mu- The polar axis of the Greenwich equatorial consists of six iron tubes eh arranged so as to form two triangular braced beams connected by us- very strong elliptical wheels of cast iron, which carry the upper dal. and lower pivots of the polar axis. These tubes are shown in section at the points T, fig. 18, which represents a section through the declination axis in the plane of the equator when the telescope is directed to a star at the equator (for the general arrangement of the mounting, see fig. 19). The driving circle is 6 feet in diameter, and turns freely on the lower pivot of the polar axis, under the action of the driving clock. The hour circle is graduated on the driving circle, and may be set to show sidereal time during other, and it is now almost exclusively employed for the mounting of modern refractors. Its essential features are (1) a comparatively short polar axis and (2) a cross-head attached to the extension of the upper pivot of the polar axis, to carry the bearings of the de- clination axis. Fig. 20- shows the Dorpat Dorpat refractor, the chef d'oeuvre of Fraunhofer, and the refractor, first aqua- J tonal of any importance that was provided with clockwork. AA is the polar axis, B the hour circle graduated on the face and read by the vernier V. C' is the driving clock, which turns an endless screw S, that gears in the toothed edge of the circle B. D is the cross head supporting at its extremities the bearings of the declination axis.

The wooden tele- scope tube rests in a strong cradle FF of cast brass flange on one end of the brass, which is screwed to a declination axis ; the declination circle EE, which is attached to its opposite end, serves to clamp the instrument in declination to the arm G. H is a weight acting on a 1 lever which presses the wheels lc (one only seen in the figure) against the upper pivot of the polar axis in order to relieve the friction of that pivot to the kindness of Mr Christie, astronomer royal. The object-glass will be actually outside the dome when the telescope is pointed near the zenith or near the horizon. The dew-cap, not shown in the model, will be always outside the dome, and it is not impossible

Type C. Many more telescopes have been made of type C than of any

Both the Dorpat and the Pulkowa refractor are defective in rigidity, especially in right ascension. The declination circle is most inconvenient of access, and slow motion in declination can only be effected when the instrument is clamped by a long and inconvenient handle, so that practically clamping in declination was not employed. The slow motion in right ascension is defective, being accomplished in the Dorpat refractor by changing the rate TELESCOPE of the clock, and in the Pulkowa refractor by a handle which when directed to any object by the circles without the observer being used affects very injuriously the rate of the clock for the time being. under the necessity to climb a special ladder. But when mach Struve's skill as an observer was such that he used to complete the larger instruments are required the hour circle becomes inaccessible bisection on the fixed wire of the micrometer by a pressure of the from the floor, and means have to be devised for readfinger on the side of the tube, - a method of proved efficiency in ing both circles from the eye end. This was first such hands, but plainly indicative of the want of rigidity in the accomplished by Grubb in the great refractor instrument and of the deficiency of the slow motions (see Mrcnoof 27-inches aperture which he constructed 40,