CHAPTER XX.

VARIOUS METHODS OF MOUNTING LARGE TELESCOPES.

We have already gone somewhat in detail into the construction of the transit circle, which is almost the most important of modern astronomical instruments. We then referred to the alt-azimuth, in which, instead of dealing with those meridional measurements which we had touched upon in the case of the transit circle, we left, as it were, the meridian for other parts of the sphere and worked with other great circles, passing not through the pole of the heavens, but through the zenith.

We now pass to the “optick tube,” as used in the physical branch of astronomy, and we have first to trace the passage from the alt-azimuth to the Equatorial, as the most convenient mounting is called.

This equatorial gives the observer the power of finding any object at once, even in the day-time, if it be above the horizon; and, when the object is found, of keeping it stationary in the field of view. But although this form is the most convenient, it is not the one universally adopted, because it is expensive, and because, again, till within the last few years our opticians were not able to grapple with all its difficulties.

Hence it is that some of the instruments which have been most nobly occupied in investigations in physical astronomy have been mounted in a most simple manner, some of them being on an alt-azimuth mounting. Of these the most noteworthy example is supplied by the forty-feet instrument erected by Sir William Herschel at Slough.

Lord Rosse’s six-feet reflector again is mounted in a different manner. It is not equatorially mounted; the tube, supported at the bottom on a pivot, is moved by manual power as desired between two high side walls, carrying the staging for observers, and so allowing the telescope a small motion in right ascension of about two hours. Our amateurs then may be forgiven for still adhering to the alt-azimuth mounting for mere star-gazing purposes.

We must recollect that, with the alt-azimuth, we are able to measure the position of an object with reference to the horizon and meridian; but suppose we tip up the whole instrument from the base, so that, instead of having the axis of the instrument vertical, we incline it so as to make the axis, round which the instrument turns in azimuth, absolutely parallel to the earth’s axis.

Of course, if we were using it at the north pole or the south pole, the axis would be absolutely vertical, as when it is used as an alt-azimuth, or otherwise it would not be absolutely parallel to the axis of the earth. On the other hand, if we were using it at the equator, it would be essential that the axis should be horizontal, since to an observer at the equator the earth’s axis is perfectly horizontal; but, for a middle latitude like our own, we have to tip this axis about 51½° from the horizontal, so as to be in proper relationship with, _i.e._ parallel to, the earth’s axis. Having done this, we can, by turning the instrument round this axis, called the polar axis, keep a star visible in the field of view for any length of time we choose by exactly counteracting the rotation of the earth, without moving the telescope about its upper, or what was its horizontal, axis. The lower circle of the instrument will then be in the plane of the celestial equator, and the upper one, at right angles to it, will enable us to measure the distance from that plane, or the declination of an object, while the lower circle will tell us the distance of the object from the meridian in hours or degrees.

With the aid of good circles and good clocks, we can thus determine a star’s position. Fig. 137 shows an Equatorial Stand, one of the first kind of equatorials used by astronomers. We see at once the general arrangements of the instrument. In the first place, we have a horizontal base, D, and on it, and inclined to it, is a disc of metal, C; again on this disc lies another disc, A, B, which can revolve round on C, being held to it by a central stud, so that when A B is in the plane of the earth’s equator its axis points to the pole and is parallel to the axis of the earth. On the upper disc there are two supports for the axis of the telescope, E, which is at right angles to the polar axis and is called the declination axis of the telescope; round it the telescope has a motion in a direction from the pole to the equator.

In the equatorial mounting, clockwork is introduced, and after the instrument has been pointed to any particular star or celestial body, the clock is clamped to the circle moving round the polar axis, and so made to drive it round in exactly the time the earth takes to make a rotation. By a clock is meant an instrument for giving motion, not with reference to time, but so arranged that, if it were possible to use it continuously, the motion would exactly bring the telescope round once in the twenty-four sidereal hours which are necessary for the successive transits of stars over the meridian.

There is an objection to the form of instrument given above,—the telescope cannot be pointed to any position near the pole, since the stand comes in the way. This is obviated in the various methods of mounting, which we shall now pass under review.

_The German Mounting._

This is the form now almost universally adopted for refractors and reflectors under 20 inches aperture.

The polar axis has attached to it at right angles a socket through which the declination axis passes, and this axis carries the telescope at one end and a counterweight at the other. The polar axis lies wholly below the declination axis, and both are supported by a central pillar entirely of iron, or partly of stone and partly of iron.

By the courtesy of Messrs. Cooke and Sons, Mr. Howard Grubb, and Mr. Browning, we are enabled to give examples of the various forms of this mounting now in use in this country for instruments of less than 20 inches aperture.

In Fig. 138, we have the type form of Equatorial Refractor introduced some 30 years ago by the late Mr. Thomas Cooke. The telescope is represented parallel to the polar axis, which is inclosed in the casing supported by the central pillar, and carries one large right ascension circle above and another smaller one below, the former being read by microscopes attached to the casing.

The socket or tube carrying the declination axis is connected with the top of the polar axis. To this the declination circle is fixed, while an inner axis fixed to the telescope carries the verniers.

The clock is seen to the north of the pillar. While this is driving the telescope, rods coming down to the eyepiece enable the observer to make any small alterations in right ascension or declination; indeed in all modern instruments everything except winding the clock is done at the eyepiece, so that the observer when fairly at work is not disturbed. The lamp to illuminate the micrometer wires is shown near the finder. The friction rollers, which take nearly all the weight off the surfaces of the polar axis, are connected with the compound levers shown above the casing of the polar axis.

In Fig. 139 we have Mr. Grubb’s revision of the German form. The pillar is composite, and the support of the upper part of the polar axis is not so direct as in the mounting which has just been referred to. There are, however, several interesting modifications to which attention may be drawn. The lamp is placed at the end of the hollow polar axis, and supplies light not only for the micrometer wires, but for reading the circles; the central cavity of the lower support is utilised for the clock, which works on part of a circle, instead of a complete one, as in the instrument already described.

In the case of Newtonian reflectors the observer requires to do his work at the upper end of the tube; this therefore should be as near the ground as possible. This is accomplished by reducing the support to a minimum. Figs. 140 and 141 show two forms of this mounting, designed by Mr. Grubb and Mr. Browning.

The two largest and most perfectly mounted refractors on the German form at present in existence are those at Gateshead and Washington, U.S. The former belongs to Mr. Newall, a gentleman who, connected with those who were among the first to recognise the genius of our great English optician, Cooke, did not hesitate to risk thousands of pounds in one great experiment, the success of which will have a most important bearing upon the astronomy of the future.

In the year 1860 the largest refractors which had been turned out of the Optical Institute at Munich under the control, first, of the great Fraunhofer, and afterwards of Merz, were those of 177 square inches area at Poulkowa and Cambridge (U.S.). Our own Cooke, who was rapidly bringing back some of the old prestige of Dollond and Tulley’s time to England—a prestige which was lost to us by the unwise meddling of our excise laws and the duty on glass,[16] which prevented experiments in glass-making—had completed a 9⅓ inch for Mr. Fletcher and a 10 inch for Mr. Barclay; while in America Alvan Clarke had gone from strength to strength till he had completed a refractor of 18½ inches for Chicago. The areas of these objectives are 67, 78·5, and 268 inches respectively.

Those who saw the great Exhibition of 1862 may have observed near the Armstrong Gun trophy two circular blocks of glass some 26 inches in diameter and about two inches thick standing on their edges. These were two of the much-prized “discs” of optical glass manufactured by Messrs. Chance of Birmingham.

At the close of the Exhibition they were purchased by Mr. Newall, and transferred to the workshops of Messrs. Cooke and Sons at York.

The glass was examined and found perfect. In time the object-glass was polished and tested, and the world was in possession of an astronomical instrument of nearly twice the power of the 18½ inch Chicago instrument—485 inches area to 268.

Such an achievement marks an epoch in telescopic astronomy, and the skill of Mr. Cooke and the munificence of Mr. Newall will long be remembered.

The general design and appearance of this monster among telescopes will be gathered from the general view given in the frontispiece, for which we are indebted to Mr. Newall. It is the same as that of the well-known Cooke equatorials; but the extraordinary size of all the parts has necessitated the special arrangement of most of them.

The length of the tube, including dew-cap and eye-end, is 32 feet, and it is of a cigar shape, the diameter at the object-end being 29 inches, at the centre of the tube 34 inches, and at the eye-end 22 inches. The cast-iron pillar supporting the whole is 19 feet in height from the ground to the centre of the declination axis, when horizontal; and the base of it is 5 feet 9 inches in diameter. The trough for the polar axis alone weighs 14 cwt., the weight of the whole instrument being nearly 6 tons.

The tube is constructed of steel plates riveted together, and is made in five lengths screwed together with bolts. The flanges were turned in a lathe, so as to be parallel to each other. It weighs only 13 cwt., and is remarkably rigid.

Inside the outer tube are five other tubes of zinc, increasing in diameter from the eye to the object-end; the wide end of each zinc tube overlapping the narrow end of the following tube, and leaving an annular space of about an inch in width round the end of each for the purpose of ventilating the tube, and preventing, as much as possible, all interference by currents of warm air with the cone of rays. The zinc tubes are also made to act as diaphragms.

The two glasses forming the object-glass weigh 144 lb., and the brass cell weighs 80 lb. The object-glass has an aperture of nearly 25 inches, or 485 inches area, and in order as much as possible to avoid flexure from unequal pressure on the cell, it is made to rest upon three fixed points in its cell, and between each of these are arranged three levers and counterpoises round a counter-cell, which act through the cell direct on to the glass, so that its weight in all positions is equally distributed among the twelve points of support, with a slight excess upon the three fixed ones. The focal length of the lens is 29 feet.

Attached to the eye-end of the tube are two finders, each of 12·5 inches area; they are fixed above and below the eye-end of the main tube, so that one may be readily accessible in all positions of the instrument. It is also supplied with a telescope having an object-glass of 33 inches area. This is fixed between the two finders, and is for the purpose of assisting in the observations of comets and other objects for which the large instrument is not so suitable. This assistant telescope is provided with a rough position circle and micrometer eyepieces.

Two reading microscopes for the declination circle are brought down to the eye-end of the main tube; the circle—38 inches in diameter—is divided on its face and edge, and read by means of the microscopes and prisms.

The slow motions in declination and R. A. are given by means of tangent screws, carrying grooved pulleys, over which pass endless cords brought to the eye-end. The declination clamping handle is also at the eye-end.

The clock for driving this monster telescope is fixed to the lower part of the pillar, and is of comparatively small proportions, the instrument being so nicely counterpoised that a very slight power is required to be exerted by the clock, through the tangent screw, on the driving-wheel (seven feet in diameter), in order to give the necessary equatorial motion.

The declination axis is of peculiar construction, necessitated by the weight of the tubes and their fittings, and corresponding counterpoises on the other end, tending to cause flexure of the axis. This difficulty is entirely overcome by making the axis hollow, and passing a strong iron lever through it having its fulcrum immediately over the bearing of the axis near the main tube, and acting upon a strong iron plate rigidly fixed as near the centre of the tube as possible, clear of the cone of rays. This lever, taking nearly the whole weight of the tubes, &c., off the axis, frees it from all liability to bend.

The weight of the polar axis on its upper bearing is relieved by anti-friction rollers and weighted levers; the lower end of the axis is conical, and there is a corresponding conical surface on the lower end of the trough; between these two surfaces are three conical rollers carried by a loose or “live” ring, which adjust themselves to equalize the pressure.

The hour-circle on the bottom of the polar axis is 26 inches in diameter, and is divided on the edge, and read roughly from the floor by means of a small diagonal telescope attached to the pillar; a rough motion in R. A. by hand is also arranged for, by a system of cogwheels, moved by a grooved wheel and endless cord at the lower end of the polar axis, so as to enable the observer to set the instrument roughly in R. A. by the aid of the diagonal telescope. It is also divided on its face, and read by means of microscopes. The declination and hour-circle will probably be illuminated by means of Geissler tubes, and the dark and bright field illuminations for the micrometers will be effected by the same means.

So soon as the success of the Newall experiment was put beyond all question by Cooke, Commodore B. F. Sands, the superintendent of the U.S. Naval Observatory, sent a deputation, consisting of Professors S. Newcomb, Asaph Hall, and Mr. Harkness, accompanied by Mr. Alvan Clarke, to examine and report upon the Newall telescope, and the result was that they commissioned Alvan Clarke to construct a large telescope for that country.

In the Washington telescope the aperture of the object-glass is 26 inches—that is, one inch larger than the English type-instrument. The general arrangements are shown in the accompanying woodcut.

It will be seen that the mounting is much lighter than in the English instrument, and a composite pillar gives place for the clock in the central cavity.

_The English Mounting._

In the _English mounting_ the telescope, like a transit instrument, has on each side a pivot, and these pivots rest on a frame somewhat larger than the telescope, pointing to the pole and supported by two pivots, one at the bottom resting on bearings near the ground, and the other carried by a higher pillar clear of the observer’s chair. The motions of the telescope are similar to those given by the German mounting in all essentials; the Greenwich equatorial is mounted in this manner. It is carried in a large cylindrical frame, supported at both ends by two pillars—above by a strong iron pillar, while the other end rests on a firm stone pillar, going right to the earth, independently of the flooring. This mounting, though preferred for the large instrument at Greenwich, has been discarded generally, as the long polar axis is necessarily a serious element of weakness; the telescope is supported on its weakest part, and it is liable to great changes from contraction and expansion of the frame.

_The Forked Mounting._

It is now getting more usual to mount Newtonians of large dimensions equatorially, in spite of the immense weight to be carried. One of the first methods was to use a polar axis in the same manner as for a refractor, only that it bifurcated at the top, forming there a fork, and between this fork the telescope is swung, after the same manner as a transit. This method of mounting was adopted by M. Foucault in the case of his first large silvered-glass reflector. The height of the bifurcation is dependent on the distance between the centre of gravity of the tube and the speculum, and if we use an extremely light tube, or if,—as it is the fashion to abolish them now altogether for reflectors,—we use a skeleton tube of iron lattice work, this bifurcation of the polar axis need not be of any great length. The polar axis being entirely below the telescope and being driven by the clock, we have a perfect method of mounting a speculum of any weight we please. This arrangement was first suggested and carried into effect by Mr. Lassell for his four-foot Newtonian, which was mounted at Malta. The polar axis was a heavy cone-shaped casting resting on its point below, and moving on its largest diameter just below the base of the fork. Lord Rosse has recently much improved upon the original idea.

As the observer must be at the mouth of the tube, he is in a very bad position as far as comfort goes, especially as the eye-end changes its position rapidly in consequence of the great length of the tube from its centre of gravity outwards. The platform on which he stands is raised on supports, extending from the floor and going up to the opening through which the telescope points to the heavens, and the whole platform is sometimes fixed to the dome of the observatory, so that it travels round with it.

With Mr. Lassell’s four-foot the observer stood in a gigantic reading box, about thirty feet high, with openings in it at different elevations. This structure was supported on a circular platform movable on rails round the base of the mounting. Almost continual variations, both of the observing height and of the circular platform, were necessary, as the distance from the centre of motion of the tube and the eyepiece was no less than 34 feet.

In Lord Rosse’s recent adaptation of this form the observer is placed in a swinging basket, at the end of an arm almost as long as the telescope tube. He is here counterpoised, and moves round a railway which surrounds the mounting at the height of the tip of the fork.

_The Composite Mounting._

There is still another form of mounting which promises to be largely used for reflectors in the future, whether the tube be lightened by its being constructed of only a framework of iron or not. This mounting is neither German nor English, but in part imitates both of these methods: hence I give it the name of Composite. There is a short polar axis supported at both ends.

Within the last few years two large reflectors have been erected, equatorially mounted in this composite manner—the great Melbourne Equatorial, constructed by Mr. Grubb, and the new Paris Equatorial, constructed by Mr. Eichens.

Of the former, Fig. 143 gives a general view, showing how the construction of this instrument differs from other equatorials which we have seen. Fig. 144 shows the mounting in more detail. C is the polar axis, T P is the declination axis, and T the small portion of the tube of the telescope, the remainder of the tube being represented by delicate lattice work, which is as light as possible, and used merely for supporting the reflector, by means of which the light is thrown back again, according to the suggestion of Cassegrain, and comes through the hole in the centre of the speculum into the eyepiece, which is seen at _y_, so that the observer stands at the bottom of the telescope in exactly the same way as if he were using a refractor.

In this enormous instrument, the tube and speculum of which alone weigh nearly three tons, the system of counterpoises is so perfect that we describe the method adopted in order to give an idea of the general arrangement of the bearing and anti-frictional apparatus. The series of weights hanging behind the support of the upper end of the polar axis are intended to take a great part of the weight of that axis off the lower support; beside which there are friction-rollers pressed upwards against the axis by the weights inside the support.

All the bearings are constructed on the same principle as the Y bearings of a theodolite—that is, the pivots rest on two small portions of their arc, 90° or 100° apart.

If allowed to rest on these bearings without some anti-frictional apparatus, the force required to move such an instrument would render it simply unmanageable and destroy the bearings.

The plan adopted by Mr. Grubb is to allow the axis to rest in its bearings with just a sufficient portion of its weight to insure perfect contact, and to support the remainder by some anti-frictional apparatus. Generally 1/50 to 1/100 of the weight is quite sufficient to allow the axis to take its bearing, and the remainder 49/50 to 99/100 can thus be supported on friction rollers, and reduced to any desired extent, without injuring in the slightest degree the perfection of steadiness obtained by the use of the Y’s. This is the plan used in the bearings of the polar axis, and the result is that the instrument can be turned round this axis by a force of 5 pounds at a leverage of 20 feet. The bearings of the declination axis are supported on virtually the same principle; but the details of that construction are necessarily much more complicated, on account of the variability of direction of the resolved forces with respect to the axis.

We may now turn to the four-foot silver-on-glass Newtonian now in course of completion at the Paris Observatory.

The illustration which we give represents the telescope in a position for observation. The wheeled hut under which it usually stands, a sort of waggon seven metres high by nine long and five broad, is pushed back towards the north along double rails. The observing staircase has been fitted to a second system of rails, which permits it to circulate all round the foot of the telescope, at the same time that it can turn upon itself, for the purpose of placing the observer, standing either on the steps or on the upper balcony, within reach of the eyepiece. This eyepiece itself may be turned round the end of the telescope into whatever position is most easily accessible to the observer.

The tube of the telescope, 7·30 metres in length, consists of a central cylinder, to the extremities of which are fastened two tubes three metres long, consisting of four rings of wrought-iron holding together twelve longitudinal bars also of iron. The whole is lined with small sheets of steel plate. The total weight is about 2,400 kilogrammes. At the lower extremity is fixed the cell which holds the mirror; at the other end a circle, movable on the open mouth of the telescope, carries at its centre a plane mirror, which throws to the side the cone of rays reflected by the great mirror.

The weight of the mirror in its barrel is about 800 kilogrammes; the eyepiece and its accessories have the same weight.[17]

It will be quite clear from what has been said that the manipulation of these large telescopes at present entails much manual and even bodily labour, and when we come in future to consider the winding of the clock, the turning of the dome, and the adjustment of the observing chair, it will be seen that the labour is enormous. To save this, in all the best instruments everything is brought to the eye-end of the telescope, movements both in right ascension and declination, reading of circles, and adjustment of illumination. Mr. Grubb has suggested that everything should be brought to this point, and that, by the employment of hydraulic power, “the observer, without moving from his chair, might, by simply pressing one or other of a few electrical buttons, cause the telescope to move round in right ascension, or declination, the dome to revolve, the shutters to open, and the clock to be wound.” He very properly adds, “This is no mere Utopian idea. Such things are done, and in common use in many of our great engineering establishments, and it is only in the application that there would be any difficulty encountered.”

_The Driving Clock._

In a previous chapter it was stated that in all large telescopes used for the astronomy of position, whether a transit circle or the alt-azimuth, what we wanted to do was to note the transit of the star across the field—the transit due to the motion of the earth; but that when we deal with other phenomena, such, for instance, as those a large equatorial is capable of bringing before us, we no longer want these objects to traverse the field, we want to keep them, if possible, absolutely immovable in the field of view of the eyepiece, so that we may examine them and measure them, and do what we please with them.

Hence it was that we found driving clocks applied to equatorials; and our description would not be complete did we fail to explain the general principles of their construction. They are instruments for counteracting the motion of the earth by supplying an exactly equal motion to the tube of the telescope in an opposite direction.

Without such a clock we may get an image of the object we wish to examine; but before we should be able to do anything with it, either in the way of measurement or observation, it would have gone from us. A glimpse of a planet or star with a large telescope will give a general notion of the extreme difficulty which any observer would have to deal with if he wanted to observe any heavenly body without a driving clock.

We can easily see at once that it would not do to have an ordinary clock regulated by a pendulum for driving the telescope, it would be driven by fits and starts, which would make the object viewed jump in the field at each tick of the pendulum. The most simple clock is therefore one in which the conical pendulum is used in the form of the governor of a steam-engine, so that when the balls A A, Fig. 146, fly up by reason of the clock driving too fast, they rub against a ring, B, or something else that reduces their velocity.

There is another form made by Alvan Clarke, in which a pendulum regulates the clock, but not quite in the ordinary way. The drawing will perhaps make it clear: A A, Fig. 147, is one of the wheels of the clock-train driving a small weighted fan, B, which is regulated so as to allow the clock to drive a little too fast. Now let us see how the pendulum regulates it. On the axis of A A is placed an arm, C, which is of such a length that it catches against the studs S S, and is stopped until the pendulum, P, swings up against one of the studs, R, which moves the piece D, like a pendulum about its spring at E, until the stud, S, is sufficiently removed to let the arm, C, pass, so that the clock is under perfect control. If, however, the arm C were fixed rigidly to the axis of the wheel A A, there would be a jerk every time C touched one of the studs. The wheel is therefore attached to the axis through the medium of a spring, F, so that when the arm is stopped the wheel goes on, but has its velocity retarded by the pressure of the spring. The pendulum is kept going in the following manner:—There is a pin fixed to the axis on the same side of the centre as C, which, as the arm approaches either stud S, raises the piece D, but not sufficiently to liberate the arm; the pendulum has then only a very little work to do to raise D and disengage the arm, C; but as soon as it is free it starts off with a jerk, due to the tension of the spring on the axis, and leaves D by means of its stud, R, to exert its full force on the pendulum and accelerate its return stroke, so that the pendulum is kept in motion by the regulating arrangement itself.

The late Mr. Cooke of York constructed a very accurately-going driving clock. This differs in important particulars from Bond’s form, though the control of the pendulum is retained.

The following extracts from a description of it will show the principal points in its construction:—

The regulator adopted is the vibrating pendulum, because amongst the means at the mechanician’s command for obtaining perfect time-keeping there is none other by which the same degree of accuracy can be obtained. The difficulty in this construction is the conversion of the jerking or intermittent motion produced by such pendulums into a uniform rotatory motion which can be available with little or no disturbing influence on the pendulum itself, when the machine is subject to varying frictions and forces to be overcome in driving large equatorials.

The pendulum is a half-second one, with a heavy bob, adjusted by sliding the suspension through a fixed slit. It is drawn up and let down by a lever and screw, the acting length of the pendulum being thus regulated.

The arrangement of the wheels represents something like the letter U. At the upper end of one branch is the scape-wheel. At the upper end of the other branch is an air-fan. The large driving-wheel and barrel are situated at the bottom or bend. All the wheels are geared together in one continuous train, which consists of eight wheels and as many pinions. The scape-wheel and the two following wheels have an intermittent motion; all the others have a continuous and uniform one.

The change from one motion to the other is made at the third wheel, which, instead of having its pivot at the end of the arbor where the wheel is fixed—fixed to the frame like the others—is suspended from above by a long arm having a small motion on a pin fixed to the frame; the pivot at the other end of the arbor is fixed to the frame as the others are, but its bole and its pivot are arranged so as to permit a very small horizontal angular motion round them, as a centre, without interfering with the action of the gearing of the wheel itself.

If the weight is applied to the clock, and the pendulum is made to vibrate, the moment it begins to move, the scape-wheel moves its quantity for a beat; the remontoire wheel, by the very small force outwardly caused by the reaction of the break-spring, relaxes its pressure against a friction-wheel, and sets at liberty the train of the clock.

The spring is now driven back to the break-wheel, but before it can produce more than the necessary friction to keep the train in uniform motion, another beat of the clock again releases it. The repetition of these actions produces a series of impulses on the break-wheel of such a force and nature as to keep the train freely governed by the pendulum.

The uniform rotatory motion obtained by this clock as far as experiments can be made by applying widely different weights, and comparing the times with a chronometer, is perfectly satisfactory.

A clock constructed on the same principle, connected, and giving motion to a cylinder, will, it is presumed, make an excellent chronograph.

The form of governor most usually employed will be seen in figures previously given. The governor raises a plate and thus becomes a frictional governor, by which all overplus of power is used up in frictions, or by that doubling the driving power no, or only a small, difference should be brought about in the rate.

Other forms of driving clocks or governors invented by Foucault and Yvon Villarceau are now being largely employed. In them the rapid motion of a fan and other devices are introduced.

A driving clock adjusted to sidereal time requires adjustments for observations of the sun and moon. This (as at _z_ in Fig. 144) is sometimes done once for all by differential gearing thrown into action by levers when required.

Mr. Grubb has lately made a notable improvement upon the usual form by controlling the motion of the governor by a sidereal clock and an electric current.

There are various methods of attaching the clock to the polar axis. One is to make the clock turn a tangent screw, gearing into a screw-wheel on the axis of the telescope, which can be thrown in and out of gear for moving the telescope rapidly in right ascension. Another method is to have a segment of a circle on the polar axis which can be clamped or unclamped at pleasure by means of a screw attached to it. A strip of metal is attached to each end of the segment and is wound round a drum turned by the clock, so that the two are geared together just as wheels are geared by an endless strap passing round them. This arrangement gives a remarkably even motion to the telescope. When the strap is wound up to the end of the segment, which is done in about two or three hours’ work, the drum is thrown out of gear and the arc pushed back to its starting-place again.

_THE LAMP._

In the description of the transit circle we saw how the Astronomer-Royal had contrived to throw light into the axis of the telescope, so that the wires were either rendered visible in a bright field, or, the field being kept dark, the wires were visible as bright lines in a dark field. That is the difference between a bright field, and a dark field of illumination. Now a bright field of illumination in the case of equatorials is managed by an arrangement as follows.—A A, Fig. 149, is a section of the tube of the telescope. Near the eyepiece is a small lamp, D, swung on pins on either side which rest on a circular piece of brass swinging on a pin at C, and a short piece of tube at E, through which the light passes into the telescope and falls on a small diagonal reflector, F. This reflects the rays downwards into the eyepiece. When the telescope is moved into any position the lamp swings like a mariner’s compass on its gimbals, and still throws its light into the tube, and the light mixes up with that coming from the star, but spreads all over the field of view instead of coming to a point, so that the star is seen on a bright field, and the wires as black lines. Now if the star which is observed is a very faint one, we defeat our own object, for the light coming from the lamp puts out the faint star.

We have seen how the illumination of the wires, instead of the field, is carried out in the Greenwich transit. The same method can be adopted in the case of equatorials, the light from the diagonal reflector being thrown on other diagonal reflectors or prisms on either side the wires in the micrometer so as to illuminate them. Messrs. Cooke and Sons have devised a lamp of very great ingenuity, Fig. 150. It is a lamp which does for the equatorial, in any position, exactly what the fixed lamp does for the transit circle. It is impossible to put it out of order by moving the telescope. There is a prism at P reflecting the light into the telescope tube, and at whatever different angle of inclination, or whatever may be the size of the telescope on which this lamp is placed, it is obvious that the lamp never ceases to throw its light into the reflector inside the telescope; and any amount of light, or any colour of light required can be obtained by turning the disc containing glass of different colours or the other having differently sized apertures, in order to admit more or less light, or give the light any colour.

In both these arrangements the lamp is hung on the side of the telescope, while Mr. Grubb prefers to hang it at the end of the declination axis, as shown in Figs. 139 and 140.

The function of the lamp then is to illuminate the wires of the micrometer eyepiece, of which more presently; but Mr. George Bidder places the micrometer itself outside the tube of the telescope, the light of a lamp being thrown on the wires.

This is done as follows:—On the same side of the wires as the lamp is a convex lens and reflector so arranged that the rays from the wires are reflected through a hole in the tube, and again down the tube to the eyepiece, where the images of the wires are brought to a focus at the same place as the stars to be measured, so that any eyepiece can be used. The wires show as bright lines in the field, and they are worked about in the field just as real wires might be by moving the wires outside the tube. A sheet of metal can be moved in front of the distance-wires so as to obstruct the light from them at any part of their length, and their bright images appear then abruptly to terminate in the field of view, so that faint stars can be brought up to the terminations of the wires and be measured without being overcome by bright lines.

Footnote 16:

It is not too much to say that the duty on glass entirely stifled, if indeed it did not kill, the optical art in England. We were so dependent for many years upon France and Germany for our telescopes, that the largest object-glasses at Greenwich, Oxford, and Cambridge are all of foreign make.

Footnote 17:

These details are given from the _Forces of Nature_ (Macmillan).