.

was afraid that, supposing these red patches on the umbra of a spot to be of the nature of the rose-coloured prominences, and observed with the polarising apparatus, the observer, in trying to distinguish them better, would turn on too much light, and lose such delicate objects.

Mr. Buckingham: I have seen Porro's reflector at Paris, and know that he uses a Nichols prism as an analyser of the light polarised by the diagonal mirror.

The titles of papers On the Nebula in Orion, by the Rev. T. W. Webb; and On the use of Prisms in preference to Plane Mirrors, for Reflectors, by Mr. Browning, were read; and the meeting adjourned.

CORRESPONDENCE.

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

COMETS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-The spherical nucleus of a comet, when it is transparent, although it is of very small actual density, is denser than the surrounding medium, and must therefore form a spherical lens; and the halo round the nucleus and the tail are merely optical phenomena produced by the sun shining through such nucleus or spherical lens. The tail must be (as it actually is) always in the direction of a ray from the sun, and if non-transparent nucleus become suddenly transparent, the tail would be projected with the velocity of light (of this instances are on record).

In the above theory I consider the tail merely a sheen or path of light (not the path of the orbit of the comet).

Two objections may arise.

1. We see paths of light in our atmosphere, but could a path of light be visible in space?

I answer this question by another. The undulatory theory is almost universally accepted: if a wave can arise in space, why not a path? which I consider to be an eddy or secondary wave (assisted perhaps by some particles floating in space).

2. Could a path of light be as brilliant as we see the tail of a comet to be as compared with the sun?

I answer, unhesitatingly, Yes; for of the brilliancy of paths of light we have abundant experimental evidence. We may take an example from the theatre, say the shadow dance in the opera of "Dinorah," where the sheen of the path of brilliant light is distinctly seen, notwithstanding the gaslights; or take an example on a larger scale, when the electric light was exhibited in the Strand, the rays collected by a concave mirror (a lens would have had nearly the same effect) were directed across the Thames, and formed a path more than half a mile long, which was visible for miles, and looked not unlike the tail of a comet.

London: Feb. 16, 1866.

I am, Sir, yours, &c.,

E. LAWRENCE.

DETERMINATION OF LONGITUDE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Your correspondent who asks how to find his longitude without trigonometry has not had the right answer as yet.

I would meet his question with another. Can he find his time? If he can, and if he has a good portable timekeeper, then the very best of all methods (saving the telegraphic) is open to him—he should carry his chronometer to some established observatory, and ask there to have the rest done for him.

But I will suppose lack of trigonometry (or rather of the power of adding and subtracting, to which by tables the necessary trigonometry is reduced) puts even his time out of his reach; still there is at least one very good method, though a sadly humiliating one for a star gazer, which is open to him: this is simply to take a good map, and with compass and ruler, and a little caution about the varying lengths of an arc of longitude, an excellent result will be obtained-one far closer to the truth than by any desultory dash at Jupiter's satellites or Moon-culminators, neither of which methods are capable of much accuracy, except after a long series of observations, or careful and doubtful allowances made.

If the equatorial must have a finger in the pie, my advice would be to place it horizontal, and use it merely to spread the map upon.

To prove that I do not speak at random, I subjoin my own early and later attempts at this discovery; the early ones when I was destitute of all instruments, the later ones with every appliance in a fixed observatory.

1851. By simple inspection of a large map of England,
long. W.

1851. By ordnance map, triangulating with a well

known station:

1. Method of Mercator

2. Method of Mid. Lats.

1860-64. By six chronometer comparisons with Liver-
pool and Greenwich, never varying as much
as one second

Concluded longitude

2 15.8

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In the Astronomical Register for January, I find Messrs. D. A. Freeman and G. J. W. recommending a querist to employ Moon-culminating stars as a means of obtaining his longitude. I regret to be compelled to say that my own advice to D. T. K. would be that of Punch to persons about to marry-Don't.

Some years ago I was myself enamoured of the principle of Moonculminators, and observed a great number of them for several consecutive months. I carefully reduced every one of these observations, and need merely inform D. T. K. that a discrepancy of 30" in longitude, as given by any two stars, was quite a common thing. My observations were made with a Troughton & Simms transit of 2.75 inches aperture, and therefore admitted of considerably greater accuracy than could ever be attained with an equatoreal. The only possible way of approximating to any trustworthy result would be to take a very large number of Moon-culminators, halving

them between opposite limbs of the Moon, so as to eliminate, as far as may be, the effect of irradiation; but it really seems to me that "Le jeu ne vaut pas la chandelle." The mischief of this method is, that a very small error in observation entails an immensely increased one in the resulting longitude.

With regard to the eclipses of Jupiter's first satellite, if D. T. K. will get some friend at Greenwich to observe them simultaneously with him, with a telescope of the same aperture and magnifying power as his own, he may thence deduce a fair latitude; but, inasmuch as every increase of aperture causes an eclipsed satellite to remain visible for a notably longer time before its final extinction, no great dependence can be placed upon a comparison with the predicted times given in the Nautical Almanac. If your correspondent really does want to determine his longitude by means of his equatoreal, it seems to me that occultations of stars by the Moon afford him the only means of doing so; but then he will assuredly require trigonometry for the reduction of his results. Why an astronomer should eschew trigonometry is a mystery to me. In fact it is hard to conceive how the possessor of an observatory can do without it for 24 hours running. I should as soon fancy that a ship could be navigated without either rudder or compass. After all, however, the equatoreal is the worst instrument that D. T. K. could employ for the determination of his geographical position. Assuming that he has a clock and a transit, he will undoubtedly do best to obtain, by careful adjustment and the observation of zenith stars, his true local time, and then compare this with Greenwich time, which he may arrange to have flashed to the nearest telegraph station, whence it might be carried by a chrono

meter.

Finally, if he will carefully read through chap. xii. of Loomis's Practical Astronomy, he will be in a position to select for himself, amid the various modes of obtaining longitude, the one best suited to his means and capability. Should he unfortunately be a stranger to that admirable work, indispensable as it is to every practical observer, I can only conclude by saying that the sooner he procures it the better.

I am, Sir, obediently yours, Forest Lodge, Maresfield, Uckfield:

Jan. 24, 1866.

SOLAR SPOTS.

WILLIAM NOBLE.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I should be glad to call the attention of some of your readers who are in possession of powerful telescopes, to the further examination of a point raised by yourself in the descriptive account of the Sun and Planets in the number of February, ult., p. 30:

"Several observers have found that spots near the limb require a different focus from those near the centre; this is, no doubt, as Dawes says, an optical deception."

I think, sir, that this point can establish a claim for further inquiry.

On February 18th, ult., I was examining the fine group of spots then visible on the Sun, and which were then rather remote from the limb. My instrument, of excellent definition, is by Troughton & Simms, 24 objectglass, 30 in. focal length, mounted equatorially. I was using a power of 40. I then turned the telescope to z Lyre, which was at a considerable altitude above the horizon, and the atmosphere very clear. I could not at first distinguish it, and found that a very considerable alteration of focus

was necessary to bring it to proper definition. From this I went back to the Sun; the spots, which before were sharp and distinct, were quite out of focus, and I had to screw the eye-piece back to the same point as before, nearer the object-glass by about 05 in. before they were in focus. I then examined Capella and the Moon, both about the same altitude as the Sun, and found that the cause was external to our atmosphere, and the peculiarity of focus was confined to the Sun. I did not on that occasion pay particular attention to the Sun's limb, and had no opportunity until yesterday to repeat the observation. Yesterday, however, I obtained precisely analogous results. A group of spots and faculæ near the limb, the limb itself, and a spot at some distance from the limb, were all brought to focus at the same point on the tube as before-Venus very near the Sun, and a Lyræ required the same point as the star focus of February 18th. The difference was, however, very slightly less, and that which I assign from yesterday's observations would be 045 in.

I then sought for a terrestrial distance, which would require a focus as much longer than the star focus as the latter was longer than the Sun focus, and established about 1,300 feet as that point. Other eye-pieces were tried, and gave similar results; only, as might be expected, the higher magnifying powers had less range of difference.

There was no spot visible near the Sun's centre; but I should think it highly probable that the focus would be much more nearly identical with the star focus. There may, however, occasionally be peculiarities arising from the elevation or depression, locally, of the refracting medium of the Sun's atmosphere, which are worthy of observation in connection with the theory of the spots.

Assuming, however, the correctness of these observations near the limb, does it not follow that the Sun, so to speak, magnifies himself, and does so about to the same amount as a lens of 1,300 feet radius reinforcing a 30inch telescope?

Dividing this distance by the focal length (2.5 feet) of the telescope, we obtain the proportion for the real diameter, compared with the apparent diameter, of 519: 520, or about 3"-6 less than the apparent diameter. Your obedient servant,

Wimbledon March 13, 1866.

F. C. PENROSE.

BINOCULAR VISION.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—In my letter on Binocular Vision with the telescope, printed in the February number of the Register, it was my intention to speak only of vision with both eyes. I endeavoured carefully to avoid making any reference to "stereoscopic effect." As, however, my friend Capt. Noble appears to have misunderstood me in reference to this matter, you will permit me, perhaps, to add the remarks which follow as a postscript to my letter.

Since the rays which reach the earth from celestial objects are sensibly parallel, it is obvious that, from the very limited distance which exists between the halves of a large object-glass divided by a suitable prism, or the separate object-glasses of a binocular instrument, no sensible dissimilarity between the images will be obtained.

If we may receive the statement of Dr. Kitchiner, in his time "the difficulty of obtaining large object-glasses induced them to make binocular telescopes. M. Aubert had one composed of two 5-foot achromatics, each having an aperture of 3 inches."

It was soon found that arming the second eye with a telescope of similar power to that furnished to the other, does not double the luminous impression conveyed to the observer. Dr. Kitchiner states that Dr. Irwin, after a variety of experiments, came to the conclusion that objects seen with both eyes appear only one-thirteenth part brighter than when seen with only one eye; also, that Mr. George Dollond and Dr. Firminger, an observer at the Royal Observatory, considered that objects viewed with a binocular telescope appear brighter than when only one telescope is used, in the proportion of 3 to 2.

Mr. Dawes' excellent observations, which are given in my former letter, prove that the employment of a second telescope does not increase the observer's power of making out the minute details of an object.

It will be, nevertheless, in accordance, I believe, with the experience of most persons, that binocular vision is much more agreeable than the use of one eye, and especially is this the case when an observation is of long duration. I have little doubt that, if we could only see as well with binocular telescopes, we should greatly prefer them. One reason may be that few of us know what to do with the eye which is not in use. Polyphemus would have made an excellent observer. It is frequently very inconvenient to place the hand before the unemployed eye if it remains open, or in any other way to screen it completely from surrounding objects. In my own case, I find it to be impossible to close the eyelids of one eye without in a slight degree impairing the vision of the other. The experiment may be made in this way: Place the hand over one eye, and look with the other at a bright point of light-for example, a street lamp-on a dark night, at a distance of two or three hundred feet; carefully notice the number and length of the rays which will appear to stream from it, then remove the hand from the other eye, and close it. I always find, under such circumstances, that the vision of the bright point is less perfect.

Most observers will, I believe, admit that even when the images received by the two eyes are not sensibly dissimilar, as is the case of those of celestial objects a very agreeable "stereoscopic effect" is frequently produced. This stereoscopic appearance will probably be observed whenever the images are of objects which are suitable for the suggestion to the mind of solidity or relief. As an example of the power of suggestion, may be mentioned the well-known experiment of causing a single photographic picture to appear strongly stereoscopic by viewing it with one eye through a small tube which cuts off from view all other objects.

By applying the binocular microscope as an eye-piece to the telescope, I have seen the craters on the Moon, Saturn, &c., appear in strong relief.

Whether the effect of the suggestions to the mind caused by the combination of two similar images may be in any case to give a truer conception of the form of a distant object, I am not prepared to say. In the instance referred to in my former letter-that of the observation of Comet II. 1862 -I thought that I received new information from using both eyes. In the "Monthly Notices" for November 1864, at page 22, Mr. John Watson says: 66 On applying it (a binocular Herschelian telescope) to the Moon, I distinctly saw black lines or rills with both eyes, which had escaped my notice with either eye when used separately. I may add, that I found it to be much more pleasant to observe with both eyes than with one, and less fatiguing."

The study of the effect of the combination of similar images may be conveniently made with a new binocular microscope recently invented by Messrs. Powell & Lealand. In this instrument each eye has the advantage of the full aperture of the object-glass, and the images are similar. In the