EXTERNAL CLUSTERS.

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distances between the different suns around us, are no more comparable than our small units on earth are with them. One of these stupendous systems is feebly represented in Plate II., as it might appear to the most powerful of our instruments; even to a good telescope it is only like a speck; but what mind shall imagine the glories—the wondrous varieties of Being-that speck must contain! Such, our earliest glance of this new perspective: System on System, of majesty unspeakable floating through that fathomless ocean: Ours, with splendours that seemed illimitable-but an unit amid unnumbered throngs; we can think of it in comparison with Creation, only as we were wont to think of one of its

own stars.

CHART BY M. STRUVE.

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acquaintance with remote regions, STRUVE confined himself within the limit of stars of the NINTH MAGNITUDE: so that Plate III., for which I am indebted to him, does not pretend to pass to the extremity of our cluster in any direction, but only to show the structure of a part of it;-it is merely the representation of the contents of a thin circular disc, whose radius, probably, attains the term of existing perfectly accurate inquiry. In relation to HERSCHEL'S original section, it is, perhaps, not more comprehensive than the sphere within the circle in the subjoined diagram; but notwithstanding

the limited nature of its pretensions, we shall find it replete with interest and profoundly suggestive.

In constructing his chart, M. STRUVE proceeded in the simplest way. By aid of the best catalogues, he first collected and arranged, according to their magnitudes and direction, all the stars whose existence and position have been recorded, up to the boundary assigned by himself; and by some curious and ingenious means, he felt

26

CHART OF OUR GALAXY.

enabled to conclude how many in each case had probably escaped observation. The numbers of the various orders thus completed, and employed according to the principle already explained (page 8), yielded an approximation to their respective distances, which are indicated on the line at the bottom of the chart; and then the astronomer proceeded to shade the various regions according to their density or richness in stars. The latter element was obtained as follows:-Taking the entire number of stars belonging to any magnitude, he supposed them equally distributed over the space belonging to that order-which gave him its mean or average density; and a comparison of the actual number of the same magnitude in any hour of right ascension, with this average, established the comparative density of such stars in that hour or direction. For instance, there are 37,739 stars of the ninth magnitude, which divided by twenty-four, gives 1572 as the average or mean density of this order-the density that would characterize each hour, if the stars had been uniformly distributed. But in the first hour, there are only 1084 of such stars; while in the sixth, we have 3318. Compared with the mean then, the density of that first hour is represented by 0.689, or about the adopted average or unit; while in the sixth hour, it is 2.11. Shadings in their proper places on the chart, of brightness proportional to the two numbers and 2.11 will