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The photographic news
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- 35.1891
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- 1891
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- Bandzählung
- No. 1724, September 18, 1891
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September 18, 1891.] THE PHOTOGRAPHIC NEWS. 653 ON THE SPACE-PENETRATING POWER OF LARGE TELESCOPES.* BY A. C. RANYARD. Unless there is some small star or dimly shining body with a large parallax which has not yet been detected, our nearest neighbour amongst the stars is the double star a Centauri. It is situated about 30° from the southern pole of the heavens, and therefore is not visible in England. The two stars together shine with a light which is a little greater than that of a first magnitude star, for the larger of these twin suns is ranked by Prof. Gould as being exactly of the 1st magnitude of the photometric scale, and the smaller star is of the 3} mag nitude. According to this photometric scale of magnitudes, which is now universally used, a star of the 1st magni tude gives just 100 times as much light as a star of the 6th magnitude. Consequently, if the larger star of the pair, which is known as a 2 Centauri, were removed to ten times its present distance, it would appear as a star of the 6th magnitude ; but this would only be the case if there were no loss of light in travelling from its more distant position. If there were any absorption of light in passing through such a vast distance of space, it might appear smaller and would probably not be visible to the naked eye, for few people see stars with their unaided eyes which are ranked as smaller than the 6th magnitude. According to the photometric scale, a star of any magnitude gives about 2} times as much light as a star of the magnitude immediately below it. Thus, a star of the 6th magnitude gives 2 -512 times as much light as a star of the 7th magnitude, and a star of the 7th magnitude gives 2'512 times as much light as a star of the 8th mag nitude. Consequently, a star of the 6th magnitude gives 6-31 times as much light as a star of the 8th magnitude, and 15'85 times as much light as a star of the 9th mag nitude, 39'81 times as much light as a star of the 10th magnitude, and 100 times as much light as a star of the 11th magnitude. Let us suppose that a2 Centauri were removed to 100 times its present distance; then, neglecting the absorp tion of light in space, it would shine as a star of the 11th magnitude of the photometric scale, and would only just be visible with a telescope of 2} in. aperture. This cal culation is based on the assumption of Prof. C. A. Young (“Text-book of General Astronomy,” sec. 822) that, for normal eyes, with a good telescope, the minimum visible for a 1 in. aperture is a star of the 9th magnitude—an estimate which about corresponds to what might be expected from the diameter of the pupil of the eye. I have measured the diameter of the pupils of several persons whom I believed to have keen sight—amongst others, the observing eyes of the Rev. T. W. Webb, Mr. Burnham, and the late Dr. H. Draper—and have found that about | in. generally corresponds to the maximum dilation of the pupil in viewing faint objects. A tele scope of 1 in. diameter would, consequently, collect about sixteen times as much light as would enter the pupil of the unassisted eye, and ought, with a suitable eye-piece, to show stars giving about one-sixteenth the light of a 6th-magnitude star just visible to the naked eye. As we have seen above, a 6th-magnitude star gives 15'85 times as much light as a 9th-magnitude star of the photometric scale. Consequently, neglecting the absorption of light * Knowledge. by the lenses, and the reflection from their surfaces, a 1 in. telescope ought, with a suitable eye-piece (which collects and sends into the pupil of the eye the whole of the light from the object-glass), to render the stars of the 9th magnitude just visible. The power used with a telescope makes some difference, as it increases the contrast between the brightness of the star and the background on which it is seen—the light of the background being dimmed by magnification, while the star in a good defining telescope is but slightly dimmed by moderate magnification. Thus, Dawes found that he could see a star of the 6th magnitude with a telescope having an aperture of only 015 in. when a power of 16} was used. In the case of the 1 in. telescope above referred to, the loss of light by absorption and reflection at the surfaces of the lenses seems to be about balanced by the increase of contrast with the background, due to the power employed. Let us suppose that « 2 Centauri were removed to a thousand times its present distance ; then, neglecting the absorption of light in travelling through space, it would appear as a star of the 16th magnitude, and would only just be visible with a telescope of 25'12 in. aperture, and if it were removed to 1585' times its present distance, it would shine as a star of the 17th magnitude of the photo metric scale, and would only just be visible in a telescope of 39 -81 in. aperture. That is, it would not be visible in the great Lick 36 in. refractor. These calculations are based on the assumption that there is no absorption of light in passing through great distances of space, and also on the assumption that there is no loss of light in passing through such thick lenses. The thickness of the object-glass of the “Washington” 26 in. refractor at its centre is nearly 8 in. ; thus, the flint glass lens is there 0'96 in. thick, while the crown glass lens is 1'88 in. thick at its centre. Such a thickness more than halves the intensity of the emergent pencil, and the loss of light by absorption in passing through the glass near the centre of the Lick object-glass must be considerable. Exact measures of the absorption of light by such great lenses would be of much interest. We may, however, probably assume with some confidence that if a2 Centauri were removed to twelve hundred times its present distance it would not be visible in the Lick telescope, even though there were no absorption of light in space, and a 2 Centauri is probably larger and brighter than our sun. (Assuming with Mr. Gore a period of seventy-seven years for this binary, and a parallax of •75 of a second, the sum of the masses of the components will be 2'14 times the mass of the sun.) Stars smaller than our sun would be lost to sight at smaller distances. Consequently, the Milky Way must either be nearer to us than a thousand times the distance of a Centauri, or the smallest stars visible in it with a telescope as large as the Washington 26 in. refractor must be larger than our sun—a supposition at which the mind rebels when we remember the vast size which this would imply for the larger stars evidently involved in or associated with the Milky Way. For example, in the Pleiades group, there are observable with the eye at the telescope a range of some 13 magnitudes of the photometric scale, which, translated into ordinary language, means that the larger stars of the cluster give more than a hundred and fifty thousand times as much light as the smaller stars of the cluster. In the photographs of the Pleiades cluster we have evidence of a range of at least 15 magnitudes, which
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