THE TRAPEZIUM IN ORION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I wish permission to ask, through the medium of the Register, what magnitude ought to be assigned to the 5th and 6th stars in the Trapezium, and what aperture a refractor requires to show both of them? Sir J. Herschel, in Art. 837 of the last edition of his work, refers to them as "two excessively minute and very close companions." The Rev. T. W. Webb, in the Intellectual Observer of November 1863, writes of the 5th as rated 13m., and of the 6th as considered by Sir J. Herschel to have only about one-third of the light of the 5th; whilst the Rev. W. A. Darby in his new work describes the 5th star as of the 15 mag. 5" distance, and adds that both 5 and 6 are shown very distinctly by a fine 74-in. aperture refractor, by Cooke of York. My instrument, a refractor by the same maker, with an aperture of 4 inches, shows the "debilissima” couple in ‹ Lyre with ease, and has shown the 5th star in the Trapezium steadily on one occasion this winter, and by glimpses at other times; and I hope to hear whether that aperture usually shows the 5th, and what aperture usually shows the 6th. I am, Sir, your obedient servant,

Menton, Alpes Maritimes:
Jan. 12, 1864.

D. A. FREEMAN.

THE OBSERVATIONS AT SOUTH VILLA.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In your notice of the Rev. W. A. Darby's Handbook, you advert to his having quoted "Bishop's " Catalogue, "Bishop's" Magnitudes, &c., as if he were not aware that Mr. Bishop, though a liberal patron of the science, was not himself the actual observer of the results to which his name was attached in his volume.

It must, I think, be acknowledged that it is not only in itself just and right, and essential to historical truth, that the name of the real observer should be attached to his observations, but also that it is very important in a merely scientific point of view, and especially so, perhaps, in the measurements of double stars. In these, personal bias is apt to vary, and sometimes to a great extent in different relative positions of the stars measured. On this account, therefore, and on the broad and just ground of "suum cuique," I beg leave to add a short statement of facts and dates.

From the date 1839'534 (July 15) to 1844°039 (Jan. 15) inclusive, every estimation of magnitude and every measurement of the double stars was made by myself, with the single exception of one set of 8 Lacerte, against which appears the remark, "Measures by Mr. Bishop."

From the date 1845°264 (April 7) and onward, the observations of double stars were, I believe, all made by Mr. J. R. Hind, who succeeded me at the South Villa Observatory; and to him we are indebted for all the remarks contained in the "Notes" appended to the measures of double stars. But wherever in the comparisons of the observations by different observers the name "BISHOP" is prefixed to

the results obtained at South Villa, the following corrections should be made in them :

From 1839 534 to 1844'039, for "BISHOP" read "DAWES." From 1845 264 and onwards, for "BISHOP" read "HIND." I remain, Sir, yours faithfully, Hopefield Observatory, Haddenham, Bucks:

Jan. 13, 1865.

W. R. DAWES.

THE MOON'S ATMOSPHERE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-If the moon's atmosphere be conceived to possess similar refractive powers to those of terrestrial transparent media, I confess my inability to accept the statement of Mr. Hopkins in the December Number of the Register. According to the laws of light, if a ray passed from a rarer medium into the denser atmosphere of the moon, it would be refracted towards the perpendicular, so that, were the stars sufficiently near to the moon's limb, refraction would cause it to dip below the limb, i.e. the ray would impinge upon the lunar surface beyond the limb, and the star would thus be cut off from our view, causing the occultation to take place before its calculated time. How much before, would obviously depend upon the extent and refractive index of the atmosphere. I do not for a moment attempt to dispute the fact quoted, "that the angle of incidence is equal to the angle of reflection." But what in the world has that to do with the question at issue?

If the projection of a star upon the moon's disc be caused by lunar atmosphere, how is it that the phenomenon is so very partially observed? I feel sure that those of your readers who are practically acquainted with the effects of irradiation will have little hesitation in referring not only "projection," but also "Bailey's beads" and "the lifting of the crescent," to that as the probable cause; for experiments at once show that it is fully competent to produce either. And yet we are told concerning projection, that "it proceeds, doubtless, from clouds or mirage on the moon ;" that "nothing else than a lunar atmosphere can satisfactorily account for the phenomena of beads," and that "the crescent is lifted doubtless from the same cause." However much I may feel inclined to thank those friends who endeavour to correct what they believe to be errors, I do enter my protest against this common practice of dogmatically stating mere possibilities or fancies.

And does any constant lunar observer amongst your readers entertain the idea that the sharp black shadows upon the dark portions of the disc at or near the terminator are due to inequalities in a seabottom, seen through water as well as an atmosphere containing clouds? If in an atmosphere like ours a sharp black shadow cannot be obtained, how much less possible is one under water! Beside this, when we consider that the "limiting angle " of rays of light incident upon water is 48°5, were there water upon the moon, similar to ours, its presence would, from this circumstance, become evident at the terminaThe idea of clouds there appears to me unsupported by any positive evidence, and is open to objections. Fearing to trespass further upon your space, I am, Sir, obediently yours,

tor.

London, S.W.

A. L. S.

THE SUN'S DIAMETER.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir, The table supplied by Mr. Howlett in your last Number, as a suitable reply to a paper of mine, was intended, I presume, to have had some other application, as it affects none of the objections raised by me to the existing methods of solar measurement.

Mr. Howlett says that on the 7th of October, at 8 A.M., at an altitude of 16°, all the diameters of the sun, as observed by him, were equal. This may well have been; for my statement was that, at a low altitude, the appearances of the sun were so entirely dependent upon the state of the atmosphere, that the sun might sink or rise either with an oval disk, or as a perfect sphere, from the varying effects of refraction. In confirmation of which let me quote a passage from the Treatise on Astronomy of a late Secretary of the Royal Astronomical Society, Mr. Thomas Galloway, F.R.S.

When the sun appears on the horizon, the rays of light which issue from the extremities of his vertical diameter are refracted unequally, on account of the difference of altitude; so that the disk, at other times circular, then assumes an oval appearance, and on measuring the horizontal and vertical diameters by means of the micrometer, the former is sometimes found to exceed the latter by four or five minutes of a degree. This effect is particularly remarkable when the sun is observed at his rising or setting from the top of a mountain or an elevation near the sea-shore, in which cases the differences of the two diameters sometimes, in particular states of the atmosphere, amount to upwards of 6', or a fifth of the whole apparent diameter of the sun.-En. Brit.

When the sun is on the meridian, at an altitude of 16°, the difference between the sun's vertical and horizontal diameters, as estimated for the Nautical Almanac, is about 7"; but our present computations are so much at fault for the sun at all altitudes, that the published Greenwich Observations for 1859 show a range of errors in the vertical diameters of the Nautical Almanac, chiefly for refraction, of nearly 9"; that is, from +461 on March 2 to 4'12 on the 24th

of May.

-

Here, while I have open before me the great volume to which I am referring, it may be useful to extract from it the figures which explain what the precise method is, followed at our Royal Observatories, for these vertical solar measurements.

Observed Vertical Diameter of the Sun for December 22, 1859.

* Horizontal and Vertical Diameters for 1859, p. 28.
+ Zenith Distances for 1859, p. 147.

It will be seen that the term "observed" given to this return is, to some extent, misleading; for the figures 781 added for refraction are not the result of observation. The Greenwich observer for refraction merely notes the states of the barometer and thermometer, and then makes with them a calculation, founded upon rules which it is tolerably certain, from our improved knowledge of the atmosphere since they were framed, and on other grounds, will soon have to be laid aside.

we are

But the question of accuracy turns more essentially upon the meaning of the words "north and south limbs." If these terms referred to fixed points of the solar orb, we should know what we were about; but it appears that by "north and south limbs to understand any parts of the sun that may happen to be highest and lowest on a given day-highest and lowest being the top and bottom of a plumb-line section, which section is a different one every moment, from the varying direction given to our plumb-line by the earth's diurnal and orbital revolutions; so that, by shifting our latitude and changing the time of observation, the sun's north pole may become our "south limb," and the sun's south pole our "north limb," or vice versa.

This may be made clear by a diagram, in which we will suppose ourselves so situate as to see the earth crossing the solar disk. On the earth let us place two observers, one observer in lat. 45° N., the other in lat. 45° S., each pointing to his zenith.

In these positions, the vertical diameter of one observer is indicated by the line a a, of the other by the line b b-the one bisecting a polar, the other an equatoreal, region of the sun. And such are, approximatively, the relations of the noon vertical and noon horizontal diameters, measured at Greenwich and the Cape. The difference of latitude being 850, the perpendiculars of the two observatories are always nearly at right angles with each other, and therefore, by all the rules of perspective and spherical trigonometry, cannot be logically compared.