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account of these matters, many of the most interesting discoveries made in recent times would be almost unintelligible, and it did not seem fitting to refer the general reader to the valuable but costly works of Roscoe and Schellen. Further, I think such a mode of treating spectroscopic analysis as I have adopted in Chapter III. of this work, more likely to be of use to the reader than a fuller but less simple account. In one respect, indeed, Chapter III. presents what is wanting in every treatise on the analysis that I have hitherto seen : the matter in pp. 128–156 exhibits what really happens when the light from the object studied is sent through a battery of prisms. In Chapter III. I give an account of the principles of Browning's automatic spectroscope, and exhibit a plan of my own by which this principle may be extended so as to include a second battery. I think that in future applications of spectroscopic analysis, the plan illustrated at p. 139 is likely to be found of considerable utility. Another large section of the book is devoted to the question of the Sun's distance. In Chapter I. will be found a very full account (the fullest popular account yet published, I believe) of the researches which have been made up to the present time into this subject: while in Appendix A the transits of 1874 and 1882 (already attracting much notice) are dealt with at length, and the best means for observing them effectively are fully considered. The subject is one to which I have given much attention. In constructing the pictures from which Plates IX. and X. have been reduced (by photolithography), every circumstance of the transit of 1874 was taken most carefully into account; and I think that I may safely say of these views, the four which accompanied them in Vol. xxix. of the “Astronomical Society's Notices,’ and Plate VIII., that they are far the most accurate graphic representations that have yet been made of this or any other transit. The views on pp. 449, 495, though small, illustrate the transit of 1882 more accurately than any views yet made, so far as I know. In Chapter II. a detailed account is given of the Sun's influence as ruler over the system of planets. Many of the relations here dealt with are novel and I think interesting. In the last three chapters I deal with the Sun's physical condition, his position as fire, life, and light of the solar system, and his place and motions among his fellow-suns. It will be seen that in Chapter VIII. I have presented reasons for considering that the most important work science has to accomplish is to show how the Sun's action can be more fully utilised than it is at present, so that before the Earth's stores of force are exhausted (as they must some day be), resources which can never be exhausted


—because unceasingly restored—may be rendered available.

It only remains for me to point out that in some of the notes I deal with matters and employ methods of treatment which would not be suitable for the main text. The general reader can omit these notes altogether, as they are not necessary for the elucidation of the subject and have only been introduced for the benefit of those more advanced students of astronomy who might desire to see certain points more thoroughly dealt with than they could be in the body of a popular treatise like the present.


LoNDoN : December 1870.

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