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Assuming the space between the eyes to be 2 inches, and the nearest distance for distinct vision to be about 10 inches, we find 15° 48' as the maximum stereoscopic angle. The possible shifting of the position of an object on the lunar disc from east to west by libration in longitude may amount to 15° 50', which is almost identical with the assumed maximum stereoscopic angle, and the displacement from north to south, by libration in latitude, never exceeds 13° 34', which falls within that angle. By the joint

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effect of a maximum libration in longitude and latitude, a point on the lunar surface may, however, be shifted nearly 21°, which is greater than that under which an object could be viewed by the eyes.

The centres of these diagrams should be 23 inches distant to give a stereoscopic picture.

An exaggerated protuberance of the central portion of the moon might result from the combination of two pictures obtained, at two epochs of maxima, in directions diagonally opposite, and the moon would appear somewhat egg-shaped. We may convince ourselves that this would be the case, by viewing, in the stereoscope, two suitably drawn orthographic projections of the lines of longitude and latitude of the sphere, especially if we purposely exaggerate the angle still more; for example, if we make the libration in latitude the double of what it is in reality.

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At the meeting at Leeds last year, there were exhibited some of my stereoscopic lunar pictures 8 inches in diameter, and an apparatus constructed expressly for viewing them. The instrument is of similar construction to Wheatstone's reflecting stereoscope; but, the objects being transparent, the usual arrangements and adjustments are considerably modified. Prisms with slight curvatures worked on their surfaces are employed, instead of mirrors, for combining the pictures which can be revolved and moved horizontally and vertically in order to place them in the true position. The effect of rotundity is perfect over the whole surface; and parts which appear like plane surfaces in a single photograph, in the stereoscope, present the most remarkable undulations and irregularities.

Light and Shade in the Photograph as compared with that of the Optical Image.-Portions of the moon, equally bright optically, are by no means equally bright chemically; hence the light and shade in the photograph do not correspond in all cases with the light and shade in the optical picture. Photography thus frequently renders details visible which escape observation optically, and it therefore holds out a promise of a fertile future in selenological researches; for instance, strata of different composition evidently reflect the chemical rays to a greater or less extent according to their nature, and may be thus distinguished t. The lunar surface very near the dark limb is copied photographically with great difficulty, and it sometimes requires an exposure five or six times as long, to bring out completely those portions. illuminated by a very oblique ray, as others, apparently not brighter, but more favourably illuminated:-the high ground in the Southern hemisphere of the moon is more easily copied than the low ground, usually called seas, which abound in the Northern hemisphere: from these circumstances I ventured, in another placet, to suggest that the moon may have an atmo*These diagrams should be 2 inches from centre to centre to give a stereoscopic picture.

† Professor Phillips has also noticed this difference between the visual and actinic brightness of portions of the lunar surface. Report of the Brit. Ass., 1853, Section A. p. 16. Monthly Notices Roy. Ast. Soc. vol. xviii. pp. 18 and 111.

1859.

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sphere of great density, but of very small extent, and that the so-called seas might be covered with vegetation. This idea respecting a lunar atmosphere has, I am inclined to believe, received some confirmation from a recent observation of Father Secchi's, that the lunar surface polarizes light most in the great lowlands and in the bottoms of the craters, and not appreciably on the summits of the mountains.

Radiating Lines in the Moon's Disc.-The mountain peak in the centre of Tycho, about one mile in height, is very distinct in the photographs, and under favourable circumstances the details in the interior of the crater are well shown. The external slopes under all illuminations are darker in the photograph than the internal walls and the bottom of the crater. Tycho would appear to have been the focus of a wonderful disturbing force which broke up the moon's crust nearly over the whole visible surface, for radiating lines converge in that conspicuous volcano, like so many circles of longitude, and cannot fail to attract attention. Several theories have been suggested to account for these radiating lines; by studying a series of photographs taken under different conditions of illumination one becomes convinced that they are due to furrows in the lunar surface*. They are in some cases overlaid by craters which must have been formed at a subsequent period; and in other cases the furrow has dislocated the crater, which must therefore have previously existed.

One very remarkable Furrow fully fifty miles broad, extending from Tycho over 45° of latitude in a north-easterly direction, is the deepest on the lunar surface. The eastern ridge of this furrow skirts Mount Heinsius, and the western ridge extends to Balliald and Euclides, where the furrow becomes very shallow, but is traceable as far as Kepler.

Another conspicuous furrow runs from Tycho in a north-westerly direction nearly up to the northern limb of the moon, and extends over 100° of latitude, passing through Menelaus and Bessel in the Mare Serenitatis through a crater (marked E in Beer and Madler's map) at the head of a promontory running into the Lacus Somniorum, when it is crossed by another furrow extending tangentially to the Apenniues. The intersection of these streaks resembles the letter X, and indicates another focus of disturbance near the crater E in north latitude 35° and west longitude 24°. The main furrow from Tycho continues on through the crater Plana, leaving Burg untouched on the east, and terminates to the south of Strabo in north latitude 60° and west longitude 45°.

A furrow best seen about the full moon or a little after, extends from Tycho, though not quite continuously, through the Mare Nectares, traversing the crater A on the west of the crater Theophilus; sweeping in a curve eastward, it leaves Tarantius on the west, and crosses the bright crater Proclus, forming an eastern tangent to Berzelius. Leaving Endymion to the south-east, it forms the southern boundary of the Mare Humboldtianum in north latitude 70° and west longitude 90°, having traversed 110 degrees of latitude.

A remarkable focus of dislocation exists in the Mare Fœcunditatis in latitude 16° south and longitude 50° west, which also, by the crossing of the lines of disturbance, looks like another letter X in the photograph.

The radiating lines of dislocation are so numerous that it would be impossible, within reasonable limits, to describe any but the principal ones; I should state, however, that they must not be confounded with the sinuous lines which radiate from Copernicus and other lunar craters, and which are markedly different in character and origin.

* Monthly Notices Roy. Ast. Soc. vol. xviii. p.111.

Value of Photography in the Production of Selenographical Charts. Pictures of Copernicus may be cited as an example of the aid photography would afford in mapping the lunar surface: this becomes especially apparent when an original negative is examined with a compound microscope. The details brought out in and around this crater in a fine negative by a three-inch object-glass are quite overwhelming from their number and variety. Not only the elaborate network of sinuous radiating lines on the exterior of Copernicus, but also the terraces in the internal walls of that wonderful volcano, the double central cone, the curvature of the sole of the crater, and its polygonal form, all appear in vigorous outline.

Again, photographs of the Apennine ridge, under different illuminations, are among the most beautiful of the results of the application of the art to selenography; it renders conspicuously evident many details of tint and form in that extensive ridge, which would escape the most careful scrutiny of the visual picture unless attention was previously directed to them by the photograph. Unaided by photography, it would indeed be almost hopeless to attempt a correct representation of that wonderful chain of mountains, affected as its form is, on account of its vast extent, by libration, and also on account of the changes in the shadows occasioned by the varying direction of the illumination. Aided by my collection of pictures, I hope to be able to acquit myself in a creditable manner of the trust I have accepted, and to contribute that quota of the lunar surface allotted to me by the British Association.

If, at a future period, the entire lunar surface is to be again mapped down, photography must play an all-important part, for, as Messrs. Beer and Mädler remarked in their invaluable work on the moon, it is quite impossible to complete even a tolerably satisfactory representation of our satellite in those rare and short moments when the mean libration occurs. One is therefore obliged to observe the moon under many different conditions of libration, and to reduce each measurement and sketch to the mean before the mapping can be proceeded with; for not only the position, but also the shape of the objects is altered by libration even from one evening to another. On the other hand, with photography at command, we may obtain in a few seconds pictures of the moon at the epochs of mean libration, and accumulate as readily a great number of records at other times. The latter would furnish, after reduction to the mean, a vast number of normal positions with which the more minute details to be seen with the telescope might be combined.

By means of a microscope, with a camera-lucida prism fixed on the eyepiece, enlarged drawings are readily made of different dimensions by varying the magnifying power and the distance of the paper from the eye-piece; with a normal magnifying power of seventeen times linear, drawings of lunar craters can be conveniently made of the exact scale used by Beer and Mädler for the large edition of their maps, by simply placing the drawing paper at the proper distance. These drawings may then be rendered more complete from time to time by filling in the minuter details by actual observation, and in this way materials accumulated for a selenographical chart such as even the skill and perseverance of a Mädler could not hope to accomplish.

Photography of the Planets.

Occasionally I take photographs of the fixed stars, and among others have made pictures of the double star Castor, but, as a general rule, I leave the fixed stars under the able custody of the Harvard Observatory, Cambridge, U.S., and devote my attention chiefly to the moon, making, however, from

time to time, photographs of the planets under the rare circumstance of a quiescent state of the atmosphere.

In photographing the planets, it is sometimes advantageous to take several pictures on the same plate; this can be conveniently done with my telescope, because the driving clock is connected with the telescope by means of a peculiar spring clutch formed of two face-ratchet-wheels. When one picture has been taken, the image is shut off, and the ratchet disconnected, so that the telescope remains at rest, the clock continuing to go. During the interval of rest, which interval is conveniently regulated by the passage of a certain number of teeth of the moving half of the clutch, the planet will have moved through a short distance in its diurnal arc; and when the clock has been again thrown into gear, the image will fall on another part of the plate. In this way, four or five images of a planet, for example Jupiter, may be obtained in a very short time. These images are arranged at equal distances along an arc of right ascension, and afford a ready means of determining the angle of position of the belts, &c., as was proposed by the late Professor Bond with respect to the angle of position of double stars.

Relation of Actinic Power to Luminosity.—I have alluded before to the difference in the optical and photographic picture of the moon; another very remarkable result of photography is the great difference which has been proved to exist in the relation of actinic power to luminosity of the various celestial objects. For example, the occultation of Jupiter by the moon, on November 8th, 1856, afforded an excellent opportunity for comparing the relative brightness of our satellite and that planet. On that occasion, Jupiter appeared of a pale greenish tinge, not brighter than the crater Plato, and, according to my estimate, of about one-third the general brilliancy of the moon; but the actinic power of Jupiter's light was subsequently found to be equal to fully four-sixths or five-sixths of that of the moon*.

Saturn required twelve times as long as Jupiter to produce a photograph of equal intensity on an occasion specially favourable for making the experiment; yet I obtained a picture of Saturn together with that of the moon in 15 seconds on May the 8th of the present year, just as the planet emerged from behind the moon's disc. The picture of the planet, although faint, is sufficiently distinct to bear enlarging.

With two pictures of the moon and a planet (or a bright fixed star) taken at a short interval at the period of an occultation, or near approach of a planet or star by the moon, we may obtain a stereoscopic picture which would make the moon (seen, of course, as a flat disc) appear nearer than the planet or star.

Stereoscopic Pictures of the larger Planets.-Photographs of the planet Jupiter, although far inferior hitherto to the optical image seen with an eyepiece, show the configuration of the belts sufficiently well to afford us the means of producing stereoscopic pictures; all that is necessary is to allow an interval to elapse between the taking of the two pictures, so as to profit by the rotation of that planet on its axis. In the space of 26 minutes the planet will have rotated through the 15° 48' necessary to produce the greatest stereoscopic effect.

Mars would, in 69 minutes, have rotated through the same angle, and, as his markings are very distinct, we may hope to obtain stereoscopic views of that planet.

The markings on the other planets are too faint to hold out a promise of similar results. Although this is the case with respect to Saturn, the ap

* Monthly Notices Roy. Ast. Soc. vol. xviii. p. 55.

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