On a subsequent evening, June 25, I repeated these comparisons, when the former observations were fully confirmed in every particular. On this evening I compared the brightest band with that of carbon in the larger spectroscope, which gives a dispersion of about five prisms.

The remarkably close resemblance of the spectrum of the comet to the spectrum of carbon necessarily suggests the identity of the substances by which in both cases the light was emitted.

The application of the Doppler-Fizeau principle to the measurement of stellar velocities has assumed great importance in astronomical investigation. It is now easy to look backward and say that this importance was inevitable, but it was not easy, half a century ago, to look forward and say that this must be so. It is characteristic of the pioneers in this field that they were slow to publish their ideas and observations.

It was Fizeau, in 1848, who first enunciated the principle correctly that motions of approach and recession must cause corresponding shiftings of the entire spectrum, including the dark lines of Fraunhofer, toward the violet and red, respectively, but without change of color. Fizeau also outlined methods for applying the principle to measuring the motions of celestial bodies toward and away from the observer. While these methods were sound theoretically, they were unpractical. All matters spectroscopic were then mysterious, and Fizeau's statements attracted no serious attention. In fact, his lecture on the subject in 1848, before a minor society in Paris, was not published until 1869. In the meantime the subject was receiving attention on the theoretical and laboratory sides from Fizeau and Clerk Maxwell, and on the stellar side from Huggins and Miller, and from Secchi. Secchi's paper in Comptes Rendus, Paris Academy, dated March 2, 1868, describes his search for high velocities of the stars in the line of sight, conducted under encouragement from Fizeau, which led to merely negative conclusions; and he remarked that success in detecting velocities in the line of sight no greater than that of the earth in its orbit would require instrumental equipment more powerful than was then at his disposal.

Almost simultaneously appeared a paper by Huggins and Miller in the Philosophical Transactions, dated April 23, 1868, from which the following paragraph is quoted:

In a paper "On the spectra of some of the fixed stars" by myself and Dr. W. A. Miller, treasurer Royal Society, we gave an account of the method by which we had succeeded during the years 1862 and 1863 in making trustworthy simultaneous comparisons of the bright lines of terrestrial substances with the dark lines in the spectra of some of the fixed stars. We were at the time fully aware that these direct comparisons were not only of value for the more immediate purpose for which they had been undertaken, namely, to obtain information of the chemical constitution of the investing atmospheres of the stars, but that they might also possibly serve to tell us something of the motions

of the stars relatively to our system. If the stars were moving toward or from the earth, their motion, compounded with the earth's motion, would alter to an observer on the earth the refrangibility of the light emitted by them, and consequently the lines of terrestrial substances would no longer coincide in position in the spectrum with the dark lines produced by the absorption of the vapors of the same substances existing in the stars.

Repeated efforts to measure the velocities of recession and approach of the stars were made in later years by Huggins and other observers; and while their results were inaccurate and erroneous, they did not work entirely in vain, for the successes of the later observers in any subject are built, to some extent, upon the failures of the pioneers. We now know that visual methods could not have had more than very moderate success, even under the most favorable conditions. The coming of very sensitive dry-plates has made it possible for a 6-inch telescope and spectrograph to measure the velocities of a greater number of stars than could be done with the 36-inch telescope, using visual methods of spectroscopy.

Perhaps Huggins's greatest contributions to the development of celestial spectroscopy have come from his efforts to interpret the original observations by means of laboratory observations made by himself and others. To these problems he brought philosophic judg ment of unusual breadth and depth. His public addresses, reviewing astronomical progress and forecasting the problems of the future, were of unusual interest and excellence. The Cardiff address of 1891 was notable in this regard.

The epoch-making work of Huggins brought him early and full recognition from universities and learned societies. His government alone was slow to reward him. He was Rede lecturer in Cambridge University in 1869; he received the degree of LL. D. from Cambridge in 1870, and the degree of D. C. L. from Oxford in 1870. He was made a member of the Royal Society in 1865. He received the Lalande gold medal and the Janssen gold medal of the Paris Academy of Sciences; the gold medal of the Royal Astronomical Society; the Royal, the Rumford, and the Copley medals of the Royal Society; the Bruce medal of the Astronomical Society of the Pacific; and perhaps others.

He received honorary degrees from many universities, and was elected to membership in the leading academies. He was president of the British Association in 1891, the year of the Cardiff meetings. He was president of the Royal Society during the years 1900–1905. On the occasion of the Diamond Jubilee of Queen Victoria, in his seventy-fourth year, he was knighted; and in his seventy-eighth year he received appointment to the Order of Merit.

It is a law of nature that ripeness must give way to youth. Fortunately, the example and work of such as Huggins live on into

succeeding generations, and the history of astronomy will keep his name on the list of great pioneers.

For 35 years he experienced able and devoted support in his scientific duties and undertakings from Lady Huggins, whose assistance was always real and active. The history of science does not tell us of more devoted coworkers than Sir William and Lady Huggins. The sympathies of all who have had the good fortune to know them go to her who has been left behind.

THE SOLAR CONSTANT OF RADIATION.1

By C. G. ABBOT,

Director of the Astrophysical Observatory of the Smithsonian Institution.

Langley once wrote:

If the observation of the amount of heat the sun sends the earth is among the most important and difficult in astronomical physics, it may also be termed the fundamental problem of meteorology, nearly all whose phenomena would become predictable if we knew both the original quantity and kind of this heat; how it affects the constituents of the atmosphere on its passage earthward; how much of it reaches the soil; how, through the aid of the atmosphere, it maintains the surface temperature of this planet; and how, in diminished quantity and altered kind, it is finally returned to outer space.

The first great advance in the study of this matter was made by Pouillet more than 70 years ago. He constructed an instrument which he called a "Pyrheliometer." It comprised a shallow circular metallic box blackened to absorb sun rays, having a thermometer inserted in the center of one circular face, and being arranged so as to expose the other circular face broadside toward the sun. The instrument was first shaded for a time, as, for instance, five minutes, then exposed to the sun an equal time, then shaded again. By reading the thermometer before and after each of the intervals just mentioned, the rise of temperature due to the sun, exclusive of the losses and gains of heat due to the surroundings, was thought to be determined. Knowing the water equivalent of the pyrheliometer and the area exposed to the sun, the result could be converted to calories per square centimeter per minute.

But it is not sufficient to know the amount of heat available in the solar beam at the earth's surface, for this is reduced by the amount of haze, dust, and water vapor in the earth's atmosphere, and even, as Rayleigh afterwards showed, diminished by the diffuse reflection of the molecules of air themselves. Hence the intensity of the solar beam not only differs from day to day, but increases between sunrise and noon, and decreases between noon and sunset, depending on the length of path of the beam in the atmosphere. Bouguer and Lambert, independently, about 1760, had derived an exponential formula connecting the intensities of the entering and outgoing beams with

1 Address by C. G. Abbot to the Solar Union Conference at the Mount Wilson Solar Observatory, California, Wednesday evening, Aug. 31, 1910.