« ПредыдущаяПродолжить »
on which the stones were placed, as on a firm foundation, is lower by several feet at a little distance from the walls, these having sheltered the foundations while the work went on rapidly on the stones above. It is only in a few places that the height of the ancient wall is indicated. Not in a single instance did I see a good specimen either of Cyclopean, polygonal, or the more finished Hellenic work detached from the wall by accident and at all recognizable. On the other hand, where, as in Santa Maura, the walls have been taken down, and the stones removed for any purpose, the weathering has not affected them beyond what is due to the time they have been exposed. Every stone that had been removed, or fallen by the progress of decay, had been entirely broken up.
Weathering on so grand a scale, and limited in this way to a period of time absolutely definite, is very rarely to be seen, and I have thought it worth while to dwell at some detail on the facts and inferences. It is one of the most interesting points in the physical geography of the Ionian Islands, and one of the most instructive examples of a great natural operation everywhere going on, that has ever fallen under my observation.
There are not wanting peculiarities of climate and other local causes that help to account for these phenomena. The nature of the limestone itself is also very favourable, the rock being in that semi-crystalline state indicative of considerable purity, and peculiarly liable to an infinite multitude of minute crevices opening a way for and even inviting the action of water. The water that falls from the air in the shape of rain is always able to effect a lodgment in such rock; vegetable matter of one kind or other soon attaches itself, and the decay of this earliest vegetation is sure to contain the material for converting the oxidized water from the clouds into an acid solution acting rapidly on the stone. Thus each successive stage facilitates and hastens the operation. The climate being insular ensures a certain supply of rain at intervals through the year, and no doubt helps to advance the work. When there is room for the intrusion of the roots of a larger kind of vegetation than that which penetrates the surface, it is extraordinary to see how the natural expansion during growth is capable of lifting out of their places even the heaviest stones, and even occasionally building such stones into the very trunk of the tree. I have noticed this especially with regard to olive-trees in Santa Maura.
BY JAMES BREEN, F.R. A. S.
HE lines of Wordsworth in which he seeks for the cause THE of his disappointment in looking through a telescopea disappointment very frequently expressed by those who look through "optic glass" for the first time-are well known :
Yet, showman, where can lie the cause? Shall thy instrument have blame
A boaster that, when he is tried, fails and is put to shame?
Or is it rather that conceit rapacious is and strong,
The bard of Rydal was, however, but an indifferent judge of what should be expected from a telescope. On a subsequent occasion, whilst gazing through one of the finest and largest instruments in the kingdom, the appearance which most attracted the "poet's eye" was that presented by a star or planet with the eye-piece out of focus, when with its coloured fringes and prismatic rings it assumed, as the poet himself expressed it, that "beautiful bird-of-paradise aspect" which is not at all popular with opticians, who much prefer distinct and colourless images.
It must, however, be confessed that many are disappointed with the telescopic appearance of the brightest single stars, which appear smaller the more perfect the instrument. It is different indeed with the larger planets, the sun, but above all the moon, which latter, with its vast and rocky mountains, particularly during its first and last quarters, is always a glorious object. In a large instrument, too, the sight presented by the resolution of one of the globular clusters, where thousands of stars are huddled into a space apparently but a few inches in diameter, is wonderful in the extreme. Perhaps, however, in all of these the amateur observer is more or less disappointed, when he hears that he is looking at those glorious orbs with powers magnifying from five hundred to a thousand times,-not considering how far they
are removed from view even when the distance of these remote objects is diminished by that amount.
When the telescope (whether that discovered by the Tuscan artist or German optician) first revealed the glories of the heavens, it was imagined that by the simple combination of an eye-lens and a single object-lens, the problem was completely solved, and that the principle at least could not be improved. It was seen indeed that it gave indistinct and coloured images, and these latter for a long course of years baffled every endeavour to correct them, and it was not until a century and a half after the invention of the telescope that they were finally removed, and that objects seen through this instrument appeared perfectly colourless and well defined. The causes of the imperfection of the original telescope were that the rays of light could not be made to meet at the same focus, and this was also the case with the colours of which the original ray of white light is composed, and which was decomposed, as by a prism, into its original parts by its passage through the lens. These are technically called aberrations or deviationsthe first, resulting from the figure of the object-lens, is termed the spherical; the latter, which depends on the glass itself (all homogeneous transparent media being affected by this cause), is called the chromatic aberration.
The spherical aberration arises from the circumstance that of the parallel rays passing through the lens A B (fig. 1), those near the margins of the glass will be bent or refracted to the focus f, whilst those passing nearer the centre of the lens will arrive at their focus at the point f'. The distance between those points is termed the longitudinal spherical aberration. If we look at the image of the sun thrown through a glass upon a dark screen, it will be noticed that instead of a point of light, the so-called focus will be surrounded by a halo. But if the outer margins of the lens be hidden by
an annular diaphragm, it will be seen that less haziness exists and the focus becomes more distinct at the point f'. By giving certain curvatures to the lens the spherical aberration may be greatly diminished, although not altogether got rid of by using a single lens, at least by one having spherical surfaces. For instance, in a plano-convex lens with the plane side turned towards the parallel rays, the distance ff' is four and a half
VOL II. NO. VIII.
times the thickness of the lens at the centre. The smallest aberration takes place when the lens used is a double convex one, the radii of the spherical surfaces being as one to six, and the more convex surface being exposed to the parallel rays, in which case the distance ff' amounts to one and one-fourteenth of the thickness of the lens. The spherical aberration can likewise be corrected by making use of elliptical and other curves instead of spherical, but the latter from their simplicity are now always preferred.
The greatest objection to the use of the refracting telescope with one lens was, however, the fringe of extraneous colour which surrounded all the objects. In a homogeneous convex lens of glass the violet rays are bent much more than the red, so that the focus of the former V (fig. 2) is situated nearer to the object-glass than the latter R, and the
other coloured rays have their foci between those two points. It has been calculated that in a lens with a focus of twentyseven feet, the distance between the foci of those two points amounts to one foot. The confusion of the coloured images resulted, and which even Newton regarded it hopeless to think of correcting, was at length got rid of by Dollond (though it had previously been found out by Hall), and is considered to be the greatest optical discovery of the eighteenth century. By making use of prisms of different liquid or solid transparent substances, it was found that the separation of the ray of white light into a coloured ribbon was not always of the same length, nor had it the same position in respect to the incident ray. From this Dollond hit upon the happy idea that it might be possible, by combining two transparent media of different dispersive powers (as these changes were called), to bend a ray of white light out of its original path without separating it into coloured rays. He found that by placing a prism of crown-glass with an angle of thirty degrees in front of another of flint-glass with an angle of nineteen degrees (the angles being opposed), that this result was completely attained; and as a lens may be considered as a series of prisms, it followed that if a convex lens of crownglass A B, (fig. 2) were placed in front of a concave lens
of flint-glass C D (with focus at E) formed to the correct curves, a colourless focus might be thus obtained. The result was the achromatic telescope. The aim of the optician was henceforward, therefore, to adopt the truest curves in which to grind and polish his glass, and to seek to unite the violet and red rays at the true focus F. Should the violet ray still be behind the red one, the telescope is under-corrected; but should the flint-glass, on the contrary, err on the opposite side, the violet ray will have a focus beyond the red one and the convex glass be over-corrected. The discovery of Dollond, it may be imagined, created great interest at home and abroad, and was a great boon to astronomy, as the celestial bodies could now be seen equally as well, if not better, with telescopes of four or five feet in length than they formerly had been with tubes of ten times those dimensions. For upwards of half a century the English achromatic held the monopoly of the market, and it is even said, led to a profitable smuggling business; and yet we believe that before the year 1828 an object-glass exceeding six inches in diameter had not been executed in this country.
The great difficulty lay in obtaining discs of glass proper for the purpose. It had not only to be pure, but beyond suspicion, and so many tests could be applied to examine its quality that very few specimens escaped without the detection of some flaw. If a skilful optician, following the advice of Mrs. Glasse, could first catch that material, the subsequent operations would give him far greater pleasure than toil, although immense labour and patience are required in grinding it in an iron basin and polishing it with mysterious powders and secret contrivances, of which each professor of the optic art has his own. The first discs of any size were those constructed by Guinand, who by his own unaided exertions produced specimens eight inches in diameter before the close of the last century, and subsequently, in conjunction with the greater genius of Fraunhofer, produced that miracle of skill-the Dorpat telescope of ten inches aperture. The successors of Fraunhofer have surpassed him in glasses ranging up to sixteen inches in diameter, which for definition and absence of colour are equal to the Dorpat, whilst exceeding it enormously in light-giving powers. What progress has been made by our own opticians might be seen at the last Exhibition. Various specimens of excellent and large foreign telescopes are to be met with in this country; indeed, the three largest of Cauchoix, of a foot in diameter and upwards, are in Cambridge, London, and Sligo.
It is not every one, however, who has an opportunity of possessing, or of looking through a telescope of these great