minutest departure from regularity will reveal itself. Even if the pencil point has been a little blunt at one part of the work, and has so made thicker strokes than elsewhere, the effect will be recognised.

Such considerations will perhaps serve to explain the occasional occurrence of a slight textural peculiarity, which does not in the least affect the main teaching of the map, a teaching which amply repays me for the 400 hours devoted to the construction of the chart.

But even this peculiarity is partly subjective. Let any one cut out a circle about four inches in diameter from a sheet of paper large enough to hide the circular boundary of the map, and place the sheet on different parts of the chart, especially where the peculiarity noticed by Mr. Backhouse seems most obvious, and it will be found that the concentricity or radiality becomes much less startling.

Dr. De la Rue has advised me to get the chart copied of its full size by the photoheliotype process. I fear, however, the cost would be too great, and it need hardly be said that any sale such a chart can have must be quite insufficient to cover the photographic expenses.

Your readers may be interested to learn that the processes of star gauging which the results of my charting led me to suggest are likely to be set on foot on a tolerably extensive scale. I have already received promises from observers using, or about to use, a 20-inch reflector, a 12-inch reflector, and an 8-inch refractor. More recruits, with these and other space penetrating powers, are wanted. There need be no fear of their work overlapping, indeed, it ought to overlap, that results may be compared. I hope next spring to complete the survey of the region covered by Taurus, Auriga, Gemini, and parts of Orion and Perseus, with the 4-inch refractor lent me for the purpose by the Astronomical Society. (Surveys with small apertures, from 3 inches upwards, are as important for completing the evidence as those with the largest.*) But the greatest encouragement I have yet received has come from the Astronomer Royal, who speaks even of applying the powers of the great equatorial at Greenwich to the gauging of the star depths at suitable times. I am satisfied that when once the work has been fairly set on foot, and results begin to be available for study, it will be quite unnecessary for me to urge any arguments in its favour.

RICHARD A. PROCTOR.

SPECTRUM OF THE ZODIACAL LIGHT.

Sir,-In your Number 115, p. 166, you report, with regard to my paper read at the Royal Astronomical Society, and describing my recent observations of the spectrum of the Zodiacal Light at Palermo, that Mr. Proctor (one of the Secretaries of the Society) "thought the observations rather doubtful, from Professor Smyth having used so many as nine prisms. With three or five he would have had a better chance upon such objects." After hearing which damaging statement the Society passed on to the next paper.

For Mr. Proctor's sake, I hope that your reporter has made some error in taking down his remark; but I am required to deal with the words

It is important that the fields of view should be made square, so that no vacant spaces may be left. I began gauging with circular fields, thinking the gaps of no importance, but having tested the results against those obtained with square fields I found the former quite untrustworthy.

actually printed by you and already misleading your readers as to the worth of the results arrived at by me, touching an astronomical phenomenon whose exact nature is at this moment a matter of keen disputation all over Europe, and throughout America as well.

Had I used so large a number as nine prisms in any manner in which a spectroscope is ever spoken of in the present day as containing nine prisms,-i. e. nine separate pieces of glass, each going on both increasing the dispersion of the previous ones, and decreasing the intensity of the transmitted light by reflection at first and second surfaces of each piece: why, of course, on such originally faint rays as those of the Zodiacal manifestation, there would have been at last at the eyepiece of the instrument so little direct transmitted light, and that so much dispersed, that the result must have been futile.

And so it would have been also with "three or five" such prisms, as stated to have been recommended to me by Mr. Proctor as better than "nine." Quite needlessly too; for I, knowing all that long before, had used only one prism of peculiar order, and which I had correctly described in the paper read to the Royal Astronomical Society as a direct "vision prism stick or combination." That one such prism was made up indeed of nine pieces of glass, but all of them, in the invariable manner of all direct-vision prisms which I have ever heard of, stuck or cemented together with Canada balsam so as to destroy reflection at the internal surfaces; while one half of the pieces being set so as to decrease the dispersion of the other half, and all of them being of very small angle, the final result was not very different to that of a single ordinary flint prism of 60°, except in the convenience of its application to the phenomenon under examination.

I trust, therefore, you will see, that it was an undeserved libel to say that I used inappropriately so many as nine "prisms," and then to undervalue my observations for that asserted reason. I may add also, that I have two direct vision prisms very similar in size and shape externally, but consisting internally, the one as above, of nine small angled, cemented pieces, and the other of three of very large angle; and that the former gives both the brighter view and the larger field, with very nearly the same dispersion. I remain, yours very truly, C. PIAZZI SMYTH.

15, Royal Terrace, Edinburgh: 2nd July, 1872.

PROFESSOR SMYTH'S OBSERVATIONS OF THE
ZODIACAL LIGHT.

Sir,-By some mistake a strange inaccuracy has crept into your report of my remarks, respecting the observations made by Professor Smyth on the zodiacal light. The report runs thus: "Mr. Proctor thought the observations rather doubtful, from Professor Smyth having used so many as nine prisms," &c. I expressed a very strong opinion of confidence in Prof Smyth's observations. I said just before concluding, that the only doubtful point seemed to be whether Prof. Smyth could not have obtained even better results had he used a direct-vision prism of five component prisms, or seven, instead of one with nine component prisms. This I suggested on the authority of our chief spectroscopist, to whom I referred by name.

But obviously, the mere fact that Prof. Smyth saw the continuous spectrum, proved that his direct-vision prism did the work it was meant to do.

I used very strong expressions in favour of the paper before sum marising the results contained in it.

The first part of the report requires also to be slightly corrected. Liais saw a faint continuous spectrum, not a strong one, and the impressions conveyed by the arrangement of the names of the observers are not quite correct. But the gentleman who reported must not be held in fault, for there was a great pressure of time, and I spoke in five minutes what I had proposed to say and read in twenty.

RICHARD A. PROCTOR,

LOGARITHM TABLES.

Dear Sir,-The paper by Mr. Lee Glaisher on Logarithm-Tables to ten figures in Monthly Notices for this month May 1872, p. 258-62, reminds me of a long cherished idea, viz., of printing only the 6th, 7th, 8th, 9th, 10th decimals of logs of every number from 1 to 108,000 in an 8vo. vol., so as to be an auxiliary supplement at moderate cost to the collections of Hutton, Bremiker, Callet, &c., now in general use.

To these might be appended the 6th to 10th decim. of log. sin. and log. tang. for every ten seconds of space; I should think the whole would not exceed in bulk the ordinary Hutton's logarithm-tables; omitting the hyperb. logs. course and dist., etc., and retaining the useful formulæ of plane and spherical trigonometry.-Ex. Gr. as in

If desired, the 6th to 10th places of differences may be printed thus on the opposite page.

I

fghik-abcde lmnpq-fghik
ghik_abcde

9

Also, the useful log. arc.log. sin.; log. tang.-log. arc. for small arcs The conversion of hyperb. to common logs., and vice versa would be much improved by a table of 001 to 999 times 2.30258 and 43429 respectively, taking out three figures value at a time, instead of two.

I have much pleasure in forwarding to you this suggestion, as you honoured me with a notice of my paper read before the Society in November last, but which, as on former occasions, the Editor of the Monthly Notices never even gave the title of; a voluntary act of suppression which the late Mr. Sheepshanks never exercised towards me or any one else. Thus e.g., I pointed out in January 'forty-four (M. N. vi. 23) the utility of simultaneous distance-measures of Mars when in opposition from the nearest stars at different terrestrial observatories (e.g., Stockholm and Cape of Good Hope) to obtain the parallax of the sun every ten years, in the stillness of night, with fixed instruments, instead of the seldom, costly, irradiation-affected transits of Venus over the solar disk.

I am, dear sir, yours truly,

74, Offord-road, Barnsbury :

S. M. DRACH.

May 29, 1872.

THE TRAPEZIUM IN THE ORION NEBULA.

Sir,-On page 37 of your last volume, you favoured me by inserting a communication having reference to the above object, and I am now desirous of adding a few words to what has already been said on the subject.

That the small stars in the interior of the Trapezium are variable seems very probable; some observers have succeeded in detecting the 5th and 6th (in the order of their brilliancy) stars with telescopes of very moderate aperture, while others have, at different times, failed to discern them. Mr. Thomas Owen, of Manchester, writes, in a letter to me that, with an object-glass of 24-inches diameter, he can frequently see five stars in the Trapezium very distinctly. In Webb's Cel. Objs., it is stated that the 5th star has been seen with a telescope of 34-inches aperture, but only on occasions when the atmosphere has been favourable for the observation of celestial objects. It would appear, therefore, that this star has, of late years, become brighter than formerly, several observers having succeeded in detecting it with instruments of only 3-inch aperture. A few years ago, there was some correspondence on this subject in the columns of the English Mechanic, several gentlemen stating that with a 3-inch refractor they had seen the 5th star on several occasions, while an anonymous correspondent disputed this, and was inclined to consider the object quite out of the reach of telescopes of the aperture stated. It would seem, however, that the star has been actually observed with a 3inch glass, if we are to believe the statements of observers whose testimony would appear to be reliable. If the smaller stars of the Trapezium are variable, they would, of course, be very difficult objects to make out when at their minimum degree of brilliancy, while, on the other hand, they would be much more conspicuous objects when at their brightest ; and, at that time, quite possibly one, if not two, of them could be observed with a 3-inch telescope or even with a smaller instrument. Mr. Owen states, that he can see the 5th very distinctly with his 23-inch, and, as he speaks so positively, it is hardly possible that he can be mistaken. In confirmation of the observation to which I have just referred, Mr. R. J. Ryle, of Burton-on-Trent, says that: "On January 6, 1872, while observing the Trapezium in Orion with a 3-in. O. G., p. 80, I was struck with a suspicion of seeing the 6th star. Knowing it requires a larger aperture I should have thought I was decidedly mistaken if I had not received a communication from a friend, at about the same time, saying, he had also suspected he could make out the same star with his three-inch telescope." It would be strange, indeed, if both these gentlemen, who are habitual observers, should have been deceived. If not, the 6th star must be very variable; for it was, according to Mr. Ryle, but very little inferior in brightness to the 5th, and was situated in precisely the same position as that assigned to it. Possibly, then, the 6th star as well as the 5th has been detected with a 3-in. O. G. If this is the case (and there does not appear much reason for doubt) the two stars referred to, must necessarily be variable, otherwise they would not have been considered such difficult objects. At any rate, there appears to be strong evidence in favour of the opinion that they are variable. Mr. John Browning, F.R.A.S, was of this opinion from observations he made in January 1867, and in March of the same year he communicated to the Register the results which he obtained. One of the stars (the 5th) which on January 25 was much brighter than another small star seen by him in the Trapezium was scarcely perceptible three days afterwards, and he was quite satisfied that

the stars had undergone a change in their brilliancy. From this, he inferred their variability, and this idea is strengthened by the other observations of which I have spoken. Certain it is that, until quite recently, the 5th star was quite out of the reach of a 3-inch O.G., and the 6th being even fainter, was scarcely visible in a telescope under 4 or 4 inches aperture, while they have now been observed (and the observation verified) with a 3-in. and 3-in. refractor. It is necessary, therefore, in order that we may become better acquainted with the extent and periods of the changes which occur in the amount of light emitted by these minute stars that they should be carefully examined on all favourable occasions. There are very many, indeed, of the readers of your valuable journal, who possess powerful instruments, and if some of them would frequently examine the Trapezium (when favourably situated for observation) their results would, no doubt, be found to be valuable. It does not appear that, during the last few years this most interesting object has been much observed by those who possess telescopes of large aperture. A systematic series of observations of the relative brilliancy of the small stars in this exquisite object (conducted on the same plan as the observations which have recently been made of the lunar crater Plato) would, no doubt, lead to our soon deciding the question as to whether or no they are variable. I hoped to be able to obtain some observations of the Trapezium myself last winter, but did not do so. Perhaps there are some amongst your subscribers who were more successful. If this is the case, perhaps they will send the observations to the Register for insertion, for there can be no doubt that the Trapezium in Orion is one of the most interesting objects for telescopic examination.

Believe me, dear Sir, your obedient servant,
WILLIAM F. DENNING,

Hon. Sec. Observing Astronomical Society.
Hollywood Lodge, Cotham Park, Bristol :
June 6, 1872.

NEW DOUBLE STAR, 6 SERPENTIS.

A few evenings since I discovered the existence of a close companion to 6 Serpentis with my 6-inch Alvan Clark refractor. This star is not included in any of the catalogues of double stars I have, and was certainly missed by the Struves. The magnitude of the companion is about the same as the companion to 5 Serpentis (10 of Struve's scale), and the distance I estimate about 3". It is not a difficult object with a six-inch aperture. The place of the star 600,870 is R.A. 15h. 14m. 25s.; N. 1° 11'. I hope astronomers will observe this fine pair, and report results with measurements or otherwise.

Chicago: June 20th.

NEW DOUBLE STARS.

S. W. BURNHAM.

I would ask the attention of observers to the following new double stars found within the last three weeks with a six-inch Alvan Clark refractor. The places are for 1870.

OPHIUCHUS. Weisse xvii, 296, 17h. 17m. 39s. ; N 13° 31'. The companion is very small at a distance of about 3" from the primary sp. The principal star is of the 8 magnitude. This is Im. 36s. preceding Σ 2159,

a wide and easy pair (D = 26", P = 326°).