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Sep 25.12.30.

CORRESPONDENCE.

N.B. We do not hold ourselves answerable for any opinions expressed by our correspondents.

THE PLANET MARS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—During the oppositions of 1862 and 1864 we have made upwards of 100 drawings of the planet Mars, an object of so much telescopic interest, from the changing phases of its very earth-like surface; and we trust that a selection from them, arranged so as to render a comparison of both series at once easy and obvious, will be a contribution not unacceptable to the Register, whilst the exhibition of "two sets made independently" will, we hope, mutually corroborate the truthfulness of their delineation. The recurrence of the markings in each series tends to the conclusion of their general permanence, modified nevertheless, apparently, by the difference of position of the planet's axis at the two oppositions, and also, at times, very considerably by atmospheric causes on its surface, as is very observable in Nos. 1 and 6 of Mr. Banks' 1862 set, which differ but a few minutes in Martial time.

Each series shows the most interesting phases at intervals during one rotation, and the Nos. of 1862 and 1864 synchronise respectively as far as practicable; but the exhibition of remarkable features has been deemed of more consequence than a rigid adherence to difference in time. There is, consequently, a greater interval between the first than the last three of each set.

The dark markings are usually supposed to be analogous to our seas; but we suggest, for further consideration, whether, when near the centre of the disc, they exhibit the characteristic reflection of a fluid surface under a vertical sun? The southern "snow zone," so conspicuous in 1862, was much less striking during the last opposition, but the northern polar region, which has since come into view, appeared to have much diffused light about it, terminated, in some views, by two luminous points, connected by a serrated outline. This is most decidedly seen in Mr. Green, No. 4, 1864, where, from irradiation, it sensibly impaired the true figure of the disc.

The drawings from which the accompanying series have been carefully reduced were, as already stated, made "independently" by Mr. Banks, at Ealing, and Mr. Green, in St. John's Wood; and they venture, from their professional familiarity with the pencil, to claim for them that truthfulness which is acquired by practice in rendering upon paper the impressions made upon the eye, whilst their fidelity to the originals has been secured by Mr. Green lithographing them himself.

The instruments employed were, by Mr. Banks, a Tulley of 3 in. aperture and 63 in. focal length; by Mr. Green, a French objectglass, of fine quality, 44 in. aperture, and 58 in. focus. The powers used were from 160 to 240; less than the former being insufficient to bring out the details; and a very few fine nights only allowing the use

of the latter. Mr. Banks found a Kitchener's Pancratic Tube, drawn out to 160, of much easier and sharper definition when observing the "half-tint markings" than a Huygenian 180, with which the bright points were generally best seen.

In conclusion, we trust that this effort to excite in others the interest we ourselves have felt will be kindly received by brother amateurs, and tend to draw their attention to the planet during the next opposition, in January 1867, when, as far at least as increased diameter is concerned, the opportunity for observation will be somewhat more favourable than during that now passed.

February 14, 1865.

We are, sir, your obedient servants,
W. L. BANKS, Ealing,

N. E. GREEN, St. John's Wood.

THE TRAPEZIUM OF ORION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Having read Mr. Freeman's interesting letter, in the last Number of the Register, I beg to be permitted to state that I have seen the 5th star in the Trapezium with a cemented object-glass of 3 in. aperture and 46 in. focus. The 6th star was readily visible to me with a fine 6 in. aperture of 104 in. focus, and I have frequently seen it steadily with an aperture of only 5 in. The visibility of these stars depends, however, very much on the state of the atmosphere and the acuteness of vision of the observer. I cannot readily perceive the fine garnet colour of the 6th star with an object-glass having an aperture much below 8 in. The glass I above referred to, of 3 in., very readily showed the "debilissima" couple in Lyræ. It may not be out of place to mention that I have frequently seen the companion to Polaris with an aperture of 1 in. (cemented glass) of 21 in. focus. I have often seen Rigel very beautifully with a similar glass, of barely 2 in. aperture to the same focal distance. I believe, however, that the Rev. Mr. Dawes distinctly sees Polaris with an uncemented glass of only 1 in. aperture. This affords a fine proof of the extraordinarily acute vision possessed by that gentleman, since it would have been impossible to have conditions of perfection in the telescope or the atmosphere beyond those I possessed with the last-mentioned small instruments; yet I could not see Polaris with the aperture cut down to 1 in., and which I reckon at least equivalent, in point of illumination, to Mr. Dawes' uncemented glass of 1% in.

Formerly, when only a deeply-coloured crown glass could be obtained for the convex of an achromatic, there was much absorption of light, and the comparative ratio of illumination between the achromatic and the reflecting telescope was much closer than it is at the present day, when colourless glasses are usually employed for the formation of an object-glass. The silvered glass reflector seems a great step in advance of speculum metal, and it would be exceedingly interesting to compare the relative illuminating power of these and the best achromatics on certain well-known objects; at any rate, it would afford me great satisfaction to know, from any of your corre

spondents possessing silvered reflectors, with how small an aperture (or, rather, area of surface, seeing that the small mirror is in the way) such objects as Polaris and Rigel are just to be steadily perceived. I am, sir, yours very truly, WILLIAM WRAY.

1 Clifton Villas, Highgate Hill, N. :

February 1, 1865.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Your correspondent, Mr. D. A. Freeman, should be able to see both the small stars in the Trapezium of Orion on a fine night, with a 4-in. glass. I have seen them easily with a 44-in. telescope; but in saying this, I would also remark that on a bad night I have failed to see either of them through my 8-in. refractor, by Cooke, of York. It is quite impossible to answer the question regarding their magnitudes until some determination is arrived at whereby a standard of magnitudes is agreed to by various astronomers. For instance, Struve's 106 mag. corresponds with Smyth's 160 mag. and Argelander's 130 mag. The magnitudes of Mr. Dawes and Struve are identical; and the method used by Mr. Dawes for determining them, which is given in the notices of the R.A.S., vol. xi. p. 191, is the simplest and most convenient of any yet proposed.

February 1865.

I

am, sir, yours faithfully,

FREDERICK BRODIE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In answer to your correspondent, Mr. D. A. Freeman, I may say that, except in extraordinarily bad weather, I can always see the 5th star in the Trapezium of 1 Orionis steadily with my object-glass of 42 inches aperture, and 61 inches focal length. It is true that it is a superb specimen of the joint work of the late Andrew Ross and of J. H. Dallmeyer, and probably very few instruments of the same size can equal it in performance. This assertion will, I conceive, be further evidenced by the fact that I have, under favourable circumstances, even seen the 6th star in the Trapezium with the same telescope. I well remember that the first occasion on which I glimpsed this extraordinarily minute object was on Friday, September 11, 1857, when observing in company with Messrs. F. Brodie and C. L. Prince, both Fellows of the Royal Astronomical Society. Mr. Brodie was the first to pick up the 6th star, but Mr. Prince and I caught it immediately afterwards. I am constrained to admit, though, that it requires a most favourable concurrence of circumstances to render so difficult an object visible with such a very small aperture.

To turn to another subject. I must say that the letter of "A. L. S." in this month's Register is very suggestive of a remark of Juvenal's, "Sed quis custodiet ipsos custodes ?" for, professing to correct Mr. Hopkins, he himself falls into a wonderful blunder. So far from a lunar atmosphere causing a star to be occulted "before its calculated time," it would actually render the star visible after the moon's limb had, in reality, covered it; and, on the other hand, at the

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