THERE is a chapter of history yet un- | fervor, theirs the faith. The harvest of hour and the man to tell the story. Then the forgotten name of O. M. Mitchell will be duly honored by American science. Not great in himself, he was the source of greatness in others. What he lacked in knowledge he made up in enthusiasm. He preached a crusade, and his followers erected domes on many a hill-top, and planted telescopes therein. His was the broad lenses was plentiful, but the efficient laborers in the observatories were few. Mr. Mitchell lectured on astronomy in many cities and towns, wherever he could get an audience, throughout the country. He was thoroughly in earnest, and therefore he interested his hearers. He told of what might be seen in the nightly skies, and every man in the audience felt a wish to become a Newton or a Herschel. | ed, and then at once proceed to make new Nothing could be simpler than his programme. Get a large telescope (the larger the better), properly housed and mount discoveries. Strange to say, there are many people yet who have no other notion of astronomy. We have not space here to tell how Mr. Mitchell brought about the building and equipping of an observatory near Cincinnati. The first subscribers to the enterprise had scarcely any money to spare, but some of them gave land, others building materials, and several assisted in the manual labor. At last the structure rose, and the telescope was bought on credit. Then certain gentlemen of Cincinnati permitted the use of their names as trustees. They took care, however, to avoid all risk and expense, and to secure free tickets to Mr. Mitchell's lectures. If the story could be fully told, you would smile at it through tears. Here is one of the stipulations: own work. The old is the classic science. Its triumphs are won by the aid of mathematics; its labors are the refinement of precision. The new study accepts with thanks all that the elder branch provides, but seeks fresh light from a different direction. Either "celestial physics" or "physical astronomy" is the title by which the young scion is now generally known, but probably our children will invent a shorter and more convenient name. Let us frankly admit that we can not call a man a "physicist" without a muscular effort, and a fear that some unlearned by-stander may think we mean a physician. The distinction between the two kinds of astronomy is well marked. One deals with the places and motions of the heavenly bodies; the other strives to ascertain the materials of sun and stars, and the effects of their radiations. It is the business of the first to say where things are; of the second to say what they are. The Allegheny Observatory, under the charge of Professor S. P. Langley, is one of the younger kind; it is de “ART. 8.—It shall be the duty of the astronomer to take charge of the observatory, and the books, instruments, and apparatus therein, belonging to the society, and preserve them as far as possible in complete order. He shall conduct a series of scientific observations such as may, in conjunction with other similar observations, conduce to new discoveries and perfect those already made in the heavens. It shall further be his duty, by himself or such assistants as he may from time to time ap-voted to "solar physics." point, to aid in gratifying the curiosity of such members of the society as may desire to examine the heavens through the telescope. He shall also deliver each year a course of lectures before such members of the society and such other citizens as may purchase a ticket to the same; the sale of these tickets to constitute his only compensation for the services rendered to the society, provided that the owner of two or more shares shall be entitled to free admittance to all such lectures." Similar enterprises, struggling with like difficulties, per aspera ad astra, sprang up in the wake of Mr. Mitchell's crusade. Many of them never reached the altitude of having a large telescope and a load of debt; very few passed that point. Among the latter was one founded in a suburb of Pittsburgh, Pennsylvania. Fair science frowned not on its humble birth. Two pieces of great good luck came to this observatory: a young astronomer of zeal and ability was made its director; a liberal-minded citizen of Pittsburgh became its helper. It has the disadvantage of overlooking the smokiest city in America. Even on the hill where the observatory stands your hands are blackened when you open a gate, and you can see little flakes of soot falling slowly through the sunshine. Astronomy has differentiated. There is the old and the new, each having its VOL. LXVII.-No. 397.-6 There are men to whom the question whether the sun is ninety-two and a half or ninety-three million miles from us is of more interest than the opportunity of hearing Patti or seeing Langtry, or even than the certainty of three meals a day. Some of these men have recently gone to the ends of the earth to observe the transit of Venus. They hope thereby to measure the sun's distance a very little more accurately. With the best possible luck, these experts in the elder branch of astronomy can only have the pleasure of helping to solve a mathematical problem. The younger science offers a more palpable kind of enjoyment. It deals with light and warmth and color. Its proofs appeal to our senses as well as to our reason. All of its experiments are delicate; some of them are exquisitely beautiful. To many of our readers, though not to those who have perused a recent work of Dr. C. A. Young, it may be as new as it was to the writer to learn that light from different parts of the solar disk is of different colors. The apparatus by which this is shown throws two patches of light side by side on a screen; they are as unlike in hue as a sapphire and a garnet. One is light from the centre of the sun's disk, and has a decidedly blue tint; the other is light from near the edge of the sun, and has a murky, chocolate color. The difference is due to the fact that rays from any part of the sun near its edge must pass to us through a much greater thickness of the solar atmosphere than rays from the centre. Draw two concentric circles, the space between them representing the sun's atmosphere, the inner circle the sun. To a point where the observer is supposed to be, outside the circles, and on a level with their centre, draw two lines from the inner circle, one from the top or bottom, the other from the middle. It will be seen at a glance that the line from top or bottom passes through a greater space between the circles than the central line. RAYS THROUGH SOLAR questions about solar light and heat, the welfare of civilized man could soon be advanced by longer strides than have resulted from the uses of steam and electricity. We are wholly dependent in many ways upon the emanations of the sun for continued existence. All our food and clothing has been made by the absorption of the solar rays. We are souls of fire and children of the sun." By whatever path we seek the laws and causes that govern climate, harvests, weather, we shall find their origin at the centre of the solar system. A complete science of the sun would enable us to foresee the years of famine or of plenty as surely as we now predict the positions of the planets. A perfect. system of weather forecasts would take the place of uncertain "probabilities." The date for the coming of the frost-king, or of the overflows when his icy fetters are broken, could be named every year far in advance. The navigator might learn before he started on a voyage what storms he could avoid, or when and where he must prepare for their en With such objects in view, the younger science is begging for help and recognition. Meanwhile the old established astronomy has all the national aid and nine-tenths of all private endowments. There is no doubt that the sun has an atmosphere, an envelope of thousands of miles in thickness. In respect to the prop-counter. erty of cutting off blue rays, and permitting red ones to pass, there is a curious similarity between the atmosphere of the sun and that of the earth. Most of us have noticed that sunlight near sunset, when the rays can only come to us through a far greater breadth of air than at noon, has a reddish tinge. It has been generally supposed that the vapor of water in the lower strata of our air has most influence on the color of the sunset rays. But this can hardly be the case with the sun's atmosphere. True, an eminent Italian savant has put on record some spectroscope observations that he regarded as showing traces of watery vapor in the envelope of the sun-a locality which we should naturally suppose to be drier than the skeletons of a medical museum. Whatever may be the explanation, the striking fact remains of this similarity of atmospheres under totally unlike conditions. Several years ago Professor Langley made the observations on difference of color, and it interested him so deeply as to give a permanent direction to his studies. He wished to learn more about the sun-rays, and of the effect upon them of absorptions by both atmospheres. If some superior being could and would confer upon us a revelation answering all In the new researches not only is there no royal road, there is not even an opened path. The way must be found by careful observation and measurement. At Allegheny experiments have been especially directed toward finding just how the sun's light and heat are modified by intervening atmospheres. In 1878-9 a systematic work was begun in measuring exactly the degree of heat in all parts of the solar spectrum. Previous studies of this kind had been made by experimenters who used the spectrum thrown by a prism. It was proposed instead to investigate the spectrum given by a "diffraction grating." There were good reasons for this choice. A "grating" is a surface of glass or of speculum metal scratched with parallel lines by means of a diamond guided by a ruling machine. The iridescent colors of mother-of-pearl, and of the wingcases of certain insects, are similarly the result of numerous lines, close together, which can be seen by the microscope. It is said that the play of colors can be transferred to white sealing-wax by simply taking an impression from the motherof-pearl. The spectroscope gratings that give the best results are ruled with from which each ray diverges from the straight eighteen thousand to thirty thousand lines line that uninterrupted light would have to the inch. Latterly some great im- taken. That distance, as may be shown provements have been made in this kind by a simple problem in geometry, is proof apparatus by better ruling and by giv-portional to the wave-length of each ray. ing the grating a slight concavity. The Prisms distribute the rays unequally. spectrum is reflected from the ruled sur- In the spectrum produced by flint-glass, face, and can either be thrown on a screen the blue end, containing the rays of short or observed with a telescope. The screen wave-length, is well displayed; but the is, of course, to be placed at the focal point red end, where are the rays of long wavewhere the concave form of the grating length, presents them crowded together. makes the reflected image brightest. The normal spectrum, on the other hand, as given by even the best of gratings, has its inconveniences. It gives several spectral images instead of one; these partly overlap, and have to be separated. In some parts of it the heat is very faint. Speaking of heat, an apology is needed. Science sometimes blunders. Until very recent years text-books have given forth the notion that there are three distinct kinds of rays emanating from the sun-light rays, heat rays, and actinic or chemical rays-and that these occupy different portions of the spectrum, though overlapping in its middle region, leaving one end bare of heat and light, and the other end bare of light and actinic power. This notion, with all that it implies, is now discarded. Dr. Young states the correct view as follows: "All the waves of solar radiation are carriers of energy, and when intercepted do work, producing heat, or vision, or chemical action, according to circumstances." It may be convenient to speak of heat rays, but it is not accurate. Actinic power has been found throughout more and more of the spectrum, by using chemicals that are duly sensitive. Cap The writer had an opportunity of seeing what could be done with an excellent grating on a day of superb sunshine at Allegheny. To those who have never looked through a spectroscope no description can give an idea of the purity and beauty of the colors which it reveals, blending them from tint to tint in an unbroken harmony. The sharpness of the Fraunhofer lines was, however, the most interesting feature of this occasion. Scarcely more than ten years ago the announcement was still made in standard treatises that between the D lines in the spectrum (which are caused by the vapor of sodium in the sun) "a fine line appears in a very perfect instrument." Soon after it was first seen this line was identified as one of the many that are produced by the vapor of nickel, and its visibility became a test of the goodness of a spectroscope. The apparatus at Allegheny, when the spectrum was thrown on a screen, showed the two D lines wide apart, with the nickel line plainly to be seen between. A small telescope was then substituted for the screen. The writer again examined the space between the D lines, and though not skilled in suchtain W. de W. Abney, of the Royal Engiwork, found without effort two lines, somewhat faint, yet distinctly visible, in the interval. Fourteen were counted by another observer. The great advantage of the diffraction spectrum over that given by a prism is that the former presents the rays in an orderly manner. Without going into abstruse details, it may be briefly stated that the image obtained from a grating has been properly called the normal spectrum, because the rays in it are dispersed equally throughout its extent, and their places in it are proportional to the length of their waves. Every spectrum is composed of rays that have been bent; that is, on leaving the prism or grating they travel in a new direction. In the normal spectrum there is a certain angular distance by The experi neers, is said to be able to photograph a |