One of the features I will touch upon very briefly, partly because this paper has already exceeded the limits I had proposed to myself when I commenced it, and partly because, without illustrations, the real nature of the peculiarity could hardly be adequately exhibited. I refer to the arrangements of the stars in streams and sprays. Here I do not speak of star-streams, such as those recognised by the ancients in the constellations Aquarius, Hydra, and Eridanus, but to minute wisps (as it were) of stars, whose figure seems associated in a significant manner with the regions in which they appear. In some parts of the charts this arrangement is so peculiar and complicated as to give the portion of the heavens represented an appearance resembling the twigwork of a large tree, as seen projected against a winter-sky.

But the teachings of the chart are not yet exhausted. Peculiarities of structure present themselves, which suggest unexpected relations among the orbs which people space. The nature of some of these peculiarities can only be understood by a reference to the chart itself; and respecting others, I forbear to speak at present, in order that at another time I may discuss them more fully than space will here permit. Perhaps the most important result of the construction of the chart is the information afforded as to the regions of the heavens which are most likely to reward the stargauger. The chart shows what regions of the heavens should be selected for this kind of survey. I believe that telescopists cannot more effectually advance our knowledge of the structure of the heavens than by joining in this muchneeded work.

meaning of the observed relations. It is not impossible that in a short time I may be able to publish the result of my inquiries.

D D

STAR UNTO STAR.

WHEN nearly twenty years ago, Drs. Huggins and Miller published the first results obtained from the spectroscopic study of stars, few could have supposed that a line of research so difficult and delicate would lead to the bold and startling views of the star-depths which now seem opening out before us. Still less would it have been thought that the method of research would be so modified that the observations belonging to it could be pursued without the direct personal study of the stellar spectra which have been found so difficult, and even (where exact researches were in question) so painful. In 1864 the observer who wished to determine whether a special substance existed in the vaporous atmosphere of a star, had to compare the spectrum of the star directly with the spectrum of the substance. In other words he had first to turn his telescope upon the star with such precision that the image of the star should fall on the fine slit of the spectroscope (and be kept there by clock motion driving the telescope, throughout the operation), and the light of the star being then sifted out by the action of the prisms in the spectroscope, so as to form a rainbowtinted streak or spectrum crossed by dark lines where certain tints are missing (on account of special absorptive action in the vaporous atmosphere of the star), the observer had to bring into the same field of view, and into precisely corresponding position, by the action of the same spectroscope, the bright line spectrum of whatever substance he wished to deal with. If the bright lines forming the spectrum of magnesium, or sodium, or calcium, or the like, were found

to correspond exactly with dark lines or missing tints in the spectrum of the star, then the observer would know that the particular substance giving those bright lines (or, more correctly, shining with those tints) existed in the atmosphere of the star. But he might very well be in doubt as to the precise accuracy of the coincidences (on which everything depends), or he might not be able to perceive clearly, yet might suspect the existence of one or other of the dark lines necessary to complete the evidence. To make sure he must cause the electric spark producing the spectrum of the substance he is dealing with to flash again and again out of the darkness, wearying the eye by the constant alternation of darkness with bright light. Not a few minutes, but many hours, on even several observing nights, would be required for each observation of the sort; and later, some other observer, with different visual powers, or with instruments of greater or less precision, might throw doubt on the accuracy. of the observation, and the whole work might have to be repeated.

Now, all this is changed. A photographic record of the spectrum is taken (hitherto only of the blue, violet, and ultra-violet part, but before long the whole visible spectrum, and parts invisible beyond the red and violet, will doubtless be photographed), and either at the same time or under precisely the same optical conditions, a photograph of the sun's spectrum (not taken directly from the sun, but either from the twilight sky or from a planet like Venus which reflects pure sunlight), and then the known dark lines in the solar spectrum can be compared directly with the dark lines in the spectrum of the star. If doubt be afterwards thrown on the result, the slips with the recorded photographic spectra are always available for comparison. And thus star after star can be added to the list of those whose light-record of their vaporous structure has been obtained. Fainter and fainter stars can be dealt with as the delicacy of sensitive plates is increased, or as the accuracy of the clock-driving of telescopes is increased, until the photographic plate may be

exposed during the whole of any clear night to receive the light impressions from a star. Already Dr. Draper has obtained records of the spectra of stars of the tenth magnitude—that is, far beyond the range of ordinary vision— though as yet such records of faint stars have not been available for the kind of research we are considering. In. fact they have only been received accidentally, so to speak, when search was being made for something entirely dif ferent.

We are not, however, here concerned to consider at any length the methods employed. It is interesting, and will appear more so as we proceed, to note how widely the research we are considering is likely to be extended in the future. But at present we propose chiefly to discuss the most remarkable result which has rewarded the method of spectroscopic inquiry into the stars, whether by ordinary -vision or by the use of photographic appliances.

The result to which we refer is the marshalling of the stars into orders, different in colour, which spectroscopic analysis shows to be due to difference in present physical constitution, which again analogical reasoning shows to be due to difference in age.

Take first the bluish-white stars of which Sirius, Vega, Altair, and others are typical.

In the first place, we note that the only star of this order whose distance has been even roughly determined (Alpha Centauri in the southern hemisphere is a yellowish-white star) is demonstrably a much larger orb than our own sun, if the quantity of light which a sun emits is any indication of size. Sirius is so remote that the motion of the earth in her vast orbit, 185 million miles in diameter, scarcely at all affects the apparent position of that brilliant star. Very exact and careful study of the star indicates apparent motion due to the earth's real motion in a tiny ellipse, the larger axis of which is roughly about the 4,000th part of the moon's apparent diameter—the nature of the observation being such that this larger axis may be as much as the 3,000th or as

little as the 5,000th part of the moon's apparent diameter, or even lie outside those limits. Taking the mean of the best measurements, a distance is inferred so great that our sun's light, were he placed at that distance, would be reduced to about the 50th part of the apparent lustre of a first-magnitude star, or, roughly, to about the 200th part of the lustre of Sirius. Hence it would follow that if an average square mile of the surface of Sirius emits as much light as an average square mile of the sun's surface, the surface of Sirius must be 200 times as large as the surface of our sun. If so, the diameter of Sirius would be about 14 times the diameter of the sun (for 14 times 14 are 196), and his volume about 2,800 times, or in round numbers 3,000 times the volume of the sun. We can hardly suppose that his volume, or probably his mass, is less than a thousand times larger than the sun's.

Of other stars of the bluish-white order we know less, with precision, but we do know so much as this, that all the brighter ones must be, and therefore even the fainter ones probably are, very much larger than the sun. For though the actual distance of Vega and Altair, for example, cannot be determined, it is because they are so far away that attempts at measurement fail. If either of them were as near as Sirius, its distance would be as readily determinable. But the measures which, applied to Sirius, give a recognisable result, fail utterly when applied to Vega and Altair. It is true, results are published in our books of astronomy which if accepted would indicate a measured distance in the case of Vega, but it is utterly untrustworthy. Vega and Altair lie beyond the range of the best methods of measurement yet invented. But noting that their lustre still exceeds many times that which the sun would have if removed to the distance of Sirius, we infer safely that the lustre of those two bluish-white stars exceeds in yet greater degree that which our sun would have if removed to their distance: in what precise degree we cannot determine, but we may confidently say that these stars are very much larger than our