« AnteriorContinuar »
in caling penti, at abosity in tharters of
at 9.43, for at that time the Moon entered the penumbra of the earth's shadow ; yet, three-quarters of an hour after this, 110 diminution of luminosity in the Moon could be detected.
At length, at about 10h. 30m., a slight shade was seen stealing gently over the Moon's disk, commencing at the limb in the N.F. quadrant. This shading steadily increased in intensity until eleven o'clock. At this time a darkening of the limb in the N.E. became distinctly perceptible. Three minutes earlier the Moon had entered the umbra of the Earth's shadow; still, though I was watching most anxiously at the telescope, I could not detect the exact instant at which this, the second stage of the eclipse, began.
The darkness kept creeping over the disk, veiling first one and then another well-known object, until it had covered about one-third of the Moon's surface. When it had reached thus far, many of the craters on the dark limb became distinctly visible. I noticed also that some of the ray-streaks projected within the shadow. At the maximum of eclipse, which occurred at 12.26, three-quarters of the disk seemed darkened; the exact amount was 0.693, the whole disk being 1.
The full disk of the Moon could at all times be dimly made out, the edge of its disk, even the portion most deeply immersed in the shadow, being much brighter than the other portions in shadow, and being, in fact, encircled by a narrow line of light. An observation in close relation to this was made during a late solar eclipse by a very careful observer, Captain Noble. This gentleman noticed that the limb of the Moon, though not the body, was visible, extending some distance beyond the solar disk. What is the meaning of this peculiar appearance? It seems the very reverse of what we might expect to obtain. At the time of an eclipse the Moon is in opposition, and it is, of course, at the full. Under these circumstances we might expect that the centre of the disk would be the most luminous, and that there would be a degradation of light towards the edges.*
The planets Mars, Saturn, and Jupiter also exhibit an increased lumincsity at the edges of their disks. This is accounted for by supposing that they are surrounded by clouds. The whole matter would be explained if we could suppose that any vapour exists in the atmosphere of the Moon. Everything, except the appearance I have been describing, points to the contrary conclusion.
A peculiarity of the light reflected from the Moon's surface,
This singular action of the edge of the disk appears to extend to the actinio raye. In one of Mr. Warren De La Rue's exquisite photographs of the full Moon, I noticed that some of the grey plains were continued to the edge of the disk. Frequently they almost touched the edge, but the extreme edge was always marked by a narrow line of light.
apparently in direct contrast with that to which I have just referred, is, that Mr. De la Rue finds that he cannot photograph with facility within some distance from the edge of the terminator. Objects that present the same luminous appearance to the eye as those in the middle of the illuminated portion of the disk, are yet only impressed with a very feeble degree of intensity upon the photographic plate.
Can these facts be of any assistance in reasoning upon the character of the surface of our satellite ? For my own part I do not think any person who has been in the habit of observing the Moon with an instrument of large aperture will be able to accept the glacial theory.
When using a telescope of ten or twelve inches aperture, protecting the eye from the great glare by a single-reflecting prism solar eye-piece, the Moon's surface is seen to be almost entirely covered with markings of a considerable variety of tone and colour, many of these frequently changing in a few days, both in shape and hue. Even with an instrument of six inches diameter, some of these changes are easily perceptible.
During the late eclipse, I devoted especial attention to the colour of the Moon. It is a generally received opinion, that when the obscuration has proceeded to the extent of two-thirds or more of the surface, the obscured portion of the Moon appears of a strong coppery red, and that the edge of the shadow on the surface appears of a very decided blue.
In Mr. Norman Lockyer's translation of Amédée Guillemin's “The Heavens,” and in Keith Johnston's “Atlas of Astronomy," diagrams of lunar eclipses are painted in chromolithography in the colours I have stated. My observations were made with Mr. Barnes's 10-inch silvered-glass speculum, furnished with a reflecting prism, and an achromatic eye-piece. During the whole time of the eclipse, I could never detect any trace of colour upon the Moon, except what I ordinarily see there. Looking through a four-inch refractor, I also observed the same freedom from colour just noted.
The colour stated to be seen on the part of the Moon under eclipse is usually ascribed to the refraction of some of the solar light as it passes through our atmosphere. I venture to suggest that, when the colour is visible, absorption plays an equally important part in producing it. This absorption would affect principally the blue rays of the spectrum, and it would be very small whenever our atmosphere is free from mist. Now, on the night of the 13th, as I have before said, the air was remarkably clear, and this may possibly enable us to account for the Moon's singular freedom from the colours which seem to have been observed upon it on previous occasions.
VOL. XII.-NO. III.
Still, as lunar eclipses are tolerably frequent, and, unlike solar eclipses, are visible over half the hemisphere of our globe, I cannot but conclude that such a total absence of colour during eclipses of considerable extent are very rare.
ON COLOURS SEEN DURING THE LUNAR ECLIPSE, SEPT. 13, WITH REMARKS ON THE PRECEDING
BY HENRY J. SLACK, F.G.S., SEC. R.M.S.
It was with much surprise I heard from Mr. Browning that his views of the late lunar eclipse through Mr. Barnes's 10 -inch telescope, so far from disclosing any decided chroinatic effects, led him to express a positive opinion as to their absence. I watched the same eclipse at intervals through a telescope similar to that of Mr. Barnes's, with a silvered mirror, but of smaller size (6}-inch), and furnished like his with a right-angled prism, to direct the cone of rays to the eye-piece. On referring to my notes I find the following :-“As shadow came over Aristarchus, that crater remained visible, and rather bright, until shadow advanced to near Eratosthenes, when it required sharp looking for to see it. The shadow was inky purple, and the sky colour in the neighbourhood dusky red. As shadow passed over different portions of the Moon, the darkness varied considerably, being much less over highly-reflecting portions than over the seas, which became very dark. After twelve the eclipsed limb grew noticeably redder, and objects likewise became gradually more visible. The red, coppery tint chiefly affected the lower part of the obscured limb, but was visible further in, gradually blending with the inky tints presented by the umbra at its advancing edge. By twenty minutes past twelve the increased visibility of obscured parts very striking. 12.26, eclipse at its height, and visibility of objects in obscured parts much greater than at an earlier period. As the Moon passed out of shadow, a brightening took place in opposite directions at the two edges of the limb, and bluish tints of brighter hue became conspicuous, contrasting with the reds.” I added to this, “Red colour less conspicuous in telescope than in opera-glass with two-inch lenses.” My wife's report of what she saw generally agreed with mine, but she noticed a greenish tint at the beginning of the eclipse in the penumbra, and did not see the blue as much as I did towards its termination. I think her eye is more sensitive than mine to the peculiar
savo eius, if of colonce ar does th
greens sometimes seen on the Moon, and which are very rarely distinct to me.
I should not have thought so much of the discrepancy between what we saw and what Mr. Browning saw about five hundred yards N.E. of us, if we had not ascertained by comparisons that our perceptions of colour and his are pretty much alike. Can it be that the difference arose chiefly from his employing a much larger aperture ? And does the eye become insensible to small quantities of red, when accompanied by a good deal of white light? Presuming the Moon during eclipse to omit red and white, or whitish-yellow, light, the larger aperture would collect more of both, but it does not follow that both would look more intense to the eye; the red might be overpowered as the white light increased, although its proportion to the white light might be the same. My operaglass, which is unusually free from chromatic errors, made the Moon look much redder than the telescope, and Mr. Barnes's instrument being much larger than piine might have still further diminished the red aspect.
In the presence of Mr. Browning I made several experiments with red objects under the microscope, such as transparent pieces of dark orange-red glass, a red leaf of a fuchsia, and a red postage-stamp. The glass, as a transparent object, changes hue considerably as the amount of white (or yellow white) lamp light thrown through it is varied. The postagestamp goes through similar changes as an opaque object, being very brilliant, and something between blood-colour and magenta, in full oblique illumination, and turning deeper and with a different hue as the amount of incident light is decreased.
To return to the Moon, we noticed, as Mr. Browning did, the remarkable distinctness and light tone of the margin of the eclipsed portion.
THE ABYSSINIAN EXPEDITION.
BY PROFESSOR D. T. ANSTED. TIe expedition now leaving the shores of England, and the armed forces preparing to penetrate Eastern Africa from the side of India under the British flag, having for their primary object the rescue of a few Europeans (only three of them Englishmen) from the clutches of an African tyrant, cannot but add something to our knowledge of a district hitherto only visited by isolated travellers at long intervals. As the army is to be accompanied by a scientific staff to report on the
geography, geology, and natural history of the countries visited, there is the more prospect of at least some return for the blood that must be spilt, and the treasure expended in the attempt.
It is not proposed here to consider the political aspect of the question involved, but as most of our readers will probably be glad to have some general notion of the physical geography of the country, we venture to collect together in a few pages a brief notice as to what Abyssinia is like, when and by whom it has been traversed, what is its climate, what are its resources, and what may probably be the difficulties the expedition will have to meet and overcome. A little information of this nature will serve to prepare us for the more accurate and extended information we may shortly expect to receive.
Abyssinia occupies an extensive tropical plateau, lying, for the most part, between a narrow strip of low land, forming the south-western shore of the Red Sea, and the important eastern branch of the Nile called the Blue Nile, whose sources were visited by Bruce. It is a compact four-sided area, lying between 9° and 16° N. lat. and 35° and 43° E. long. The coast of the Red Sea fringing it, and now partly occupied by Turkey, is about 600 miles in length from the Straits of Bab-el-Mandel to the northern extremity of Abyssinia. The other boundaries are Nubia to the north and west, and the Galla tribes towards the south. Within the country there are numerous streams, all feeders of the Nile; those to the north entering far to the north by the Albara, one of its main tributaries, and those to the south running directly into the Blue Nile. There are two lakes, one (Lake Tsana or Dembea) about sixty miles long, full of islands and abounding with fish. This is near the south-western part of the country, and not far from Gondar, the capital. It is also close to the sources of the Blue Nile. It is surrounded by a wide expanse of flat lands highly cultivated. The other lake is less important for its size, .and is said to consist of a large extinct volcanic crater.
The shores of the Red Sea are low, swampy, and unhealthy, but the breadth of the low land is small, nowhere exceeding 100 miles, and towards the northern part of Abyssinia not more than sixty. There are several bays and roadsteads along the coast, but they atford few good harbours for large vessels, nor are they much known. Of the whole number Massowah is considered to be the one most available for the purposes of the proposed expedition. It is situated almost at the northernmost extremity of the country, and is eminently unhealthy in summer, but fortunately the distance from safe and healthy highlands is extremely small.*
* Of modern travellers in Abyssinia, Bruce was one of the earliest and most adventurous. He crossed from Sennar on the Blue Nile eastwards to Gondar.