Following these rules which he had laid down, he made a sort of rough sketch of what he supposed might be the shape of the Milky Way. He thought it was somewhat flat, extending to a good distance breadthways and a much greater distance lengthways, and he placed our sun not far from the middle. This imagined shape of the Milky Way is called “The Cloven-disk Theory.” To explain the appearance of the Milky Way in the sky, Herschel supposed it to be cloven or split through half of its length, with a black space between the two split parts. It seems that Herschel did not hold strongly to this idea in later years, and doubts are now felt whether the rules on which he formed it have sufficient foundation. For how do we know that the stars of the Milky Way are, as a rule, much the same in size? Certainly the planets of the Solar System are very far from being uniform, and the few stars whose weight can with any certainty be measured, seem to vary considerably. There is a great difference also between the large and small suns in many of the double-stars! Again, how do we know that the bright stars are, as a rule, the nearest, and the dim stars the farthest off? Here, also, late discoveries make us doubtful. Look at Sirius and 61 Cygni--Sirius the most radiant star in the heavens, and 61 Cygni almost invisible to the naked eye. According to this rule, 61 Cygni ought to lie at an enormous distance beyond Sirius. Yet in actual fact, Sirius is the farthest away of the two. The illustrious Herschel believed that he had penetrated, on one occasion, into the star-cluster on the sword-hand in the constellation of Perseus until he found himself among sidereal depths, from which the light could not have reached him in less than four thousand years. The distance of those stars had not, and has not, been mathematically measured. Herschel judged of it by their dimness, by the strong power needed to make them visible, and by the rules which he had adopted as most likely true. Again, when Herschel found black spaces in the heavens almost void of stars, and believed that he had reached the outside borders of the Milky Way, he may have been in the right, or he may have been mistaken. The limit might lie there, or thousands more of small stars might extend in that very direction, too far off for their little glimmer to be seen through the most powerful telescope. [Illustration: A CLUSTER OF STARS IN PERSEUS.] If this latter idea about the Milky Way being formed of a great many brilliant suns, and of vast numbers of lesser suns also, be true, astronomers will, in time, be able to prove its truth. For in that case, many faint telescopic stars being much nearer to us than bright stars of the greater magnitudes, it will be found possible to measure their distance. The journey of our earth round the sun must cause a seeming change in their position between summer and winter.