For land objects, which must appear erect, a telescope is formed with additional lenses, which make a second picture, as in fig. 3. The lens (AB)

makes the first picture, and the

two lenses (CD

and EF) cross

the rays again, and make a se

M

B

Fig. 3.

cond picture, which is upright. This is viewed by the eye-piece (GH). To increase the power of these telescopes, the object-glass (AB) is made with a very long focus, or so as to form its picture as far off from itself as possible. This requires its shape to be very much flattened, and still to preserve a perfect roundness-a matter difficult of execution. All lenses are more or less imperfect; that is, the picture they form is liable to be somewhat confused, which takes off from the advantage of the instrument. The greatest evil is one that cannot be cured by a single lens-that is, the fringing or colouring of the picture. But this action has been done away with by using a double lens, or two lenses of different kinds of glass joined together. The difference in the quality of the glasses to produce colour is so managed, that they neutralise one another, and a picture free from coloured and indistinct edges is produced. This compound lens is called achromatic, or wanting in colour. With these lenses very perfect telescopes are made of 2, 3, 6, or 10 feet of length, and with eye-pieces of half or quarter of an inch, and under, of focal distance. A three-feet telescope-that is, a telescope where the picture is made 36 inches from the object-glass, and an eye-piece that lets the picture come within half an inch of the eye (a half-inch eye-piece), would magnify 72 times each way, and have the same effect as if the distance of the original were divided by 72.

A hollow or concave mirror, if the hollow be truly spherical, has the same power as a convex lens in creating a new picture of an object, that will serve the purposes of a telescope. A plane mirror also makes a picture, but it has not the power of yielding a telescopic effect with an eye-piece. The concave mirror has the advantage

of not creating a coloured picture: it is, therefore, ex

•r

tensively used

for telescopes,

especially for

those of the

S

Fig. 4.

largest power. The form of a reflecting telescope must be different from the other, that the observer may not stand between

6

the object and the mirror. Fig. 4 is a reflecting telescope, called the Gregorian, from its constructor, James Gregory of Aberdeen. The object mirror (TS) is seen at the bottom of the tube. An external object (AB), sending its rays into the tube, has a reflected picture of itself formed at IK. This picture passes on to a second small mirror (L M), which reflects it back again through a hole in the large mirror to the eye-tube, where picture cd, that the eye is to look at, is formed, and viewed close by the eyepiece (S).

Sir William Herschel got over the difficulty of placing the observer out be

tween the object and the mirror, by a simpler arrangement, represented in fig. 5. He gave the mirror (AB) at the bottom of the tube a

slight slope, so

A

that it sends its B

image to a, at

Fig. 5.

с

D

E

the edge of the tube's mouth, where it is viewed by the eyepiece without bringing the observer's head between the thing viewed and the mirror.

On this principle Sir William Herschel constructed telescopes of gigantic dimensions and power. His greatest was 40 feet long, and the mirror 4 feet wide. The use of a large mirror is to take in more light, which is apt to fail in using high magnifying powers. With an eye-piece of an inch focus, the power of such a telescope would be 480 each way, which would magnify a surface nearly a quarter of a million of times. The moon would be seen by such a power as if she were brought within 500 miles of us, her real distance being 240,000 miles.

But Lord Rosse has surpassed Herschel in the construction of reflecting telescopes. The chief difficulty in making monster telescopes (apart from the stupendous machinery for supporting and moving them), is the forming of the mirror or speculum, which is of metal, and requires to have a surface of high polish and reflecting power, and at the same time to be ground into a perfectly spherical or rather parabolic form. The mixing of the ingredients to make a good shining metal, and the casting of an immense mass, like a millstone, of an even hardness throughout; and lastly, the grinding, shaping, and polishing of the surface, make a series of operations of the utmost difficulty. After succeeding in the manufacture of the speculum, Lord Rosse has gone on to construct two telescopes of immense power -the one 26 feet long; the other, "the monster telescope," 56

feet, the focal length of the mirror being 52 feet. The 56 feet tube is 7 feet wide; the mirror at the bottom is 6 feet wide, with a glittering polish all over the surface, and weighs three tons. This largest instrument not only surpasses Herschel's and all others in size, but in the perfection of its finish, and of the picture that it produces, which is of far more consequence than the largeness of its focal range. As it is unquestionably one of the greatest curiosities of the age, both in point of construction and effect, we transfer the following description from "Chambers's Edinburgh Journal"-referring at the same time to the vignette illustration which accompanies this paper.

VISIT TO THE ROSSE TELESCOPE.

First by railway to Kildare, and thence by coach, the journey of seventy-eight miles from Dublin was satisfactorily performed; and early in the afternoon of an October day, 1846, I found myself comfortably seated in a hotel in Parsonstown-adjoining the noble mansion of the same name, where was situated the interesting object of my journey.

The appearance of Parsonstown was somewhat surprising. In travelling towards it from Dublin, various extensive tracts of bog-land are crossed, useful no doubt for fuel to the adjacent inhabitants, but otherwise unproductive, and too large and unsightly to inspire pleasing emotions. There is also seen not a little wayside poverty-mud hovels, a poor state of husbandry, and a straggling, though far from dense population. Immediately beyond one of the dreary bog regions, we come suddenly upon Parsonstown, almost as neat and brisk a town as could be seen in England-environs consisting of numerous villas, a square of good houses, and several regular streets; pretty nearly the whole being whitewashed, and possessing an air of substantiality and comfort. To account for these agreeable features, I was informed that the town has become a favourite resort of families seeking a place of genteel retirement, though the nature of the climate is by no means favourable to those requiring a dry and equable condition of the atmosphere. Perhaps not the least irresistible of its attractions are the easy terms on which building-ground is obtained from Lord Rosse, and the not less marked liberality of that nobleman in opening his extensive pleasure-grounds for several hours daily to all who choose to visit them.

A walk along a terrace of houses, forming, with the trees opposite, a species of boulevard, conducts to the gate of his lordship's domain; and, uninterrupted, we soon reach the lawn in front of the castle-a large and commodious building of some antiquity, which endured a siege during the wars of the Revolution. In whatever manner the edifice was surrounded with defences in those troublesome times, it is now open to the parklike lawn that bounds it on the north, and in which are situated

the various telescopes of its ingenious proprietor. Previous to the arrival of the steward, who was kindly deputed to afford me every desirable information, I had an opportunity of taking a general glance at the apparatus, and of looking around the grounds, through which flows, in many a meandering turn, the pretty little river Lower Brusna, a tributary of the Shannon.

The telescopes, on which of course my attention was mainly riveted, are three in number, like the degrees of comparisongreat, greater, greatest-and are all situated near to each other, so as to command a fair view of the heavens over the tops of the trees which bound the lawn. The smallest is contained in a dome-roofed edifice, resembling an ordinary observatory, and therefore presents nothing exteriorly remarkable. The two larger are under no roof; they are open to the weather; great black tubes dangling from chains like the funnels of steamboats, lowered slopingly from the perpendicular. One of these is 26 feet long and 3 feet in diameter, and is adjusted so as to wheel round to point in any required direction. The other, which is appropriately called the “monster telescope,” measures 50 feet in length by 6 feet in diameter, and is suspended between high and substantial walls, which permit its command of only a stripe of the heavens from south to north-an arrangement which, however imperative from the bulk of the machine, I was sorry to think must somewhat lessen its usefulness.

So much for a first glance of these wonderful astronomical instruments. Before I had walked round them, the steward, an intelligent and obliging young man, placed himself at my service; and, by way of beginning at the beginning, conducted me to the workshops where the whole apparatus was made. The road proceeded through a clump of trees, and emerged on a courtyard on the right of the castle, where an entire engineering establishment disclosed itself. It was certainly something new to find a smelting furnace in active operation, blown by a steamengine, within a dozen feet of the drawing-room window of a nobleman's castle! The furnace was puffing away at a great rate; and a neat little engine was diligently occupied not only in blowing the bellows, but in giving motion to sundry shafts, belts, and pulleys. A large complex piece of machinery, designed to turn and smooth the specula of the telescopes, was at rest; and about a dozen men were here and there occupied with sundry minor operations. All the workmen who have, from first to last, been engaged in preparing the telescopes, or the apparatus connected with them, have been natives-a fact which will seem strange to those who are unacquainted with the aptitude for instruction of the Irish character. It will probably appear not less surprising that the instructor and superintendent of these artisans in their multifarious duties has been no other than Lord Rosse, whose accomplishments in practical science, independently of his rank, would place him in a distinguished posi

tion. As illustrative of his skill in this respect, an anecdote is related by the good folk of Parsonstown, to the effect of his lordship having on one occasion visited an engineering establishment in London, and there shown such a knowledge of mechanics, that the proprietor, in his ignorance of whom he was addressing, offered him a situation of some hundreds per annuma compliment, one can fancy, which is not likely to be paid to many other members of the peerage. Devoted to pursuits involving mathematical calculations, he has been pretty constantly engaged, since 1826, in perfecting the means of telescopic observation; and on this interesting branch of science alone he is understood to have spent, till the present time, as much as £30,000. Long-continued and costly as have been these labours, they could not have realised their present successful results, unless they had been conducted with the most imperturbable patience and good-humour, together with a readiness to have recourse to new and hazardous expedients on all occasions of difficulty and defeat. On this account, the operations of his lordship more resemble the long and studious exercises of the old alchemists in their laboratories, than the proceedings of a modern man of science and letters.

It would be a very long story to tell all that Lord Rosse has done since he commenced his labours twenty years ago, and I need therefore refer only to the more important steps in his operations-a slight popular sketch being alone desirable in these pages. His lordship began by attempting to make a telescope, with glass lenses, of the old and usual kind. A short course of experiments proved that little good could arise from this effort, and he then adopted the principle of the reflecting telescope, [described in the preceding pages.] The power of a reflecting speculum depends, like that of a lens, on its diameter and degree of sphericity; or, properly speaking, its capacity for collecting the rays which stream from any object. In employing the exact spherical concavity, however, there is always a slight confusion to the eye, in consequence of the centre of the speculum giving the image or reflection of the object at a different focal distance from the parts at and near the circumference. This confusion, which is called spherical aberration, can be avoided only by forming the speculum with a parabolic curve-that is, a concavity slightly elliptical or oval; but the exceeding difficulty of producing this figure with mathematical accuracy, may be judged from the fact, that if two specula of six feet in diameter -the one spherical, and the other parabolic-were pressed into contact at the centre, the edges would not diverge from each other more than the thousandth part of an inch!

Lord Rosse, at the outset, abandoned the spherical form altogether, and endeavoured to produce a true parabolic speculum, which should be free from aberration. An approximation to the parabolic in small specula had previously been attained by cer