readily found from its position as indicated in the figure, or it may be identified by following the curved line indicated by B, 8, e and in the Great Bear.

The constellation of Virgo is principally characterised by the first magnitude star called Spica, or a Virginis. This may be found from the Great Bear; for if the line joining the two stars a and y in that constellation be prolonged with a slight curve, it will conduct the eye to Spica. We may here notice another of those large configurations which are of great assistance in the study of the stars. There is a fine equilateral

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triangle, whereof Arcturus and Spica form two of the corners, while the third is indicated by Denebola, the bright star near the tail of the Lion (Fig. 76).

In the summer evenings when the Crown is overhead, a line from the Pole Star through its fainter edge continued nearly to the

southern horizon, encounters the brilliant red star Cor Scorpionis, or the Scorpion's Heart (Antares), which was the first star mentioned as having been seen with the telescope in the daytime.

The first magnitude star, Vega, in the constellation of the Lyre, can be, perhaps, most readily found at the corner of a bold triangle, of which the Pole Star and Arcturus form the base (Fig. 77). The brilliant whiteness of Vega cannot fail to arrest the attention, while the small group of neighbouring stars which form the Lyre produces one of the best defined constellations.

Near Vega is another important constellation, known as the Swan or Cygnus. The brightest star will be identified as the vertex of a right-angled triangle, of which the line from Vega to the Pole Star is the base, as shown in Fig. 78. There are in Cygnus five principal stars, which form a constellation of rather remarkable form.

The last constellation which we shall here describe is that of Aquila or the Eagle, which contains a star of the first magnitude, often known as Altair; this group can be readily found by a line from Vega over B Cygni, which passes near the line of three stars, forming the characteristic part of the Eagle.

We have taken the opportunity to indicate in these sketches of the constellations the positions of some other remarkable telescopic objects, the description of which we must postpone to the following chapters.

CHAPTER XIX.

THE DISTANT SUNS.

Comparison between the Sun and the Stars-Sirius Contrasted with the SunStars can be Weighed, but not Measured-The Companion of Sirius-Determination of the Weights of Sirius and his Companion-Dark Stars-Variable Stars-Enormous Number of Stars.

WIDE indeed is the contrast between the splendour of the noon-day sun and the feeble twinkling of even the brightest of the stars. This contrast, so forcible to our ordinary observation, can be subImitted to the test of actual measurement. Let us take the most brilliant star, Sirius. We can determine by experiment the number of times by which the light of Sirius is surpassed by the light of the sun. It is true we cannot make that comparison directly. In the bright daylight Sirius cannot even be seen, much less can his light be measured. But we can take the full moon as an intermediate step between the glory of the sun and the feeble twinkle of Sirius. It has been found that the light from the sun is about 600,000 times as great as the light from the full moon. Sirius and the full moon can be compared together, and by suitable photometers the quantity of light from the two bodies can be measured. It can be shown that the light of about 33,000 stars, equal to Sirius, would produce an illumination equal to that of the moon. We have thus the necessary figures for comparing the brilliancy of the sun with the brilliancy of Sirius, and the result is indeed significant. The light from the sun is about twenty thousand million times as great as the light from Sirius.

But do these figures truly represent the relative importance of the sun and Sirius? Recollect where we are placed. Our earth is

near the sun; it is very far from Sirius. Our earth is not properly placed for an impartial comparison between the splendour of the sun and the splendour of Sirius. To make such a comparison, the earth should be placed midway between the two bodies, so that we could look at Sirius on one side and the sun on the other, under precisely similar circumstances. Into such a position our earth never has come, and never will come. How, then, is the real comparison to be accomplished? In this, as in many other cases, it is possible to determine by calculation what it is impossible to ascertain by experiment. It has been found, by observation, that Sirius is about one million times as far from us as the sun. If we take the distance of Sirius from the earth, and sub-divide it into one million equal parts, each of these parts would be long enough to span that great distance of 92,700,000 miles from the earth to the sun. If, therefore, the earth were to be placed half-way between the sun and Sirius, the earth would be five hundred thousand times as far away from the sun as it is at present. What would be the effect of this change upon the light received from the sun? Take the light of a candle at one foot as unity, then at two feet the light is reduced to one-fourth, and at three feet to one-ninth, and so on. The light received from a luminous source varies "inversely as the square of the distance." Transplanted to a distance 500,000 times as great as our present distance from the sun, to what a vast extent must his radiance be diminished! present our sun is as bright as 20,000,000,000 stars each equal to Sirius; but viewed from this central point, the light is reduced to an almost incredible extent. Our sun would have totally lost his pre-eminence, and would only send to us one-twelfth part of the light which we now receive from Sirius. On the other hand, by shifting our station from the vicinity of the sun to a point halfway to Sirius, the light of Sirius will become intensified in the ratio of four to one. Assuming, then, that the earth was placed in the correct position for testing the comparative splendour of Sirius and the sun, we should find, first, that Sirius was increased four-fold, and that the sun was so enormously reduced in intensity as to look only one-twelfth of the original brightness of Sirius. The con

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