NATIONAL ACADEMY OF SCIENCES S AMUEL ALFRED M ITCHELL 1874—1960 A Biographical Memoir by C . G . Abb OT Any opinions expressed in this memoir are those of the author(s) and do not necessarily reflect the views of the National Academy of Sciences. Biographical Memoir COPYRIGHT 1962 NATIONAL ACADEMY OF SCIENCES WASHINGTON D.C. SAMUEL ALFRED MITCHELL April 29, i8j4-February 22, i960 BY C. G. ABBOT AMUEL ALFRED MITCHELL, son of John Cook Mitchell, was born S at Kingston, Ontario, April 29, 1874. He died at Bloomington, Indiana, February 22, i960. His father (a tenth child) emigrated to Canada from Austell, Cornwall. His mother, Sarah Chown, born in Kingston, was the tenth child of parents who emigrated to Canada from Devonshire. Alfred was the sixth of the ten children born to his parents. Well grounded in the 3 R's, at twelve years of age he passed to the Kingston Collegiate Institute. With advanced standing in math- ematics, Latin, French, and German, and honored by receiving the Governor General's scholarship in general proficiency, he went on to Queen's University. Here, through slender, he took part in col- lege sports. In his junior year came his first acquaintance with as- tronomy, which was taught by Reverend James Williamson, a brother-in-law of Sir John A. MacDonald. The subject of astronomy came very naturally after two years of mathematics with Professor Nathan F. Dupuis and two years of physics. Queen's then had a small wooden observatory, with two instru- ments on loan from the Royal Astronomical Society of London, a transit instrument, and a 6-inch Clark equatorial. There were two clocks, a sidereal clock and a mean-time clock built by Professor Dupuis, who had been an expert clock maker before he came to be a Professor of Mathematics. SAMUEL ALFRED MITCHELL 255 At eighty, Professor Williamson, affectionately called Uncle Billy, found it pleasant and convenient to delegate the care of these instru- ments to young Mitchell. Thus he early came to know something of the technique of an astronomical observatory. Mitchell received the degree of Master of Arts in 1894, and was also awarded the medal in mathematics. In the following session he was appointed Tutor in Physics, continuing in charge of the observa- tory, and in the next session was appointed Instructor in Mathe- matics. But Professor Dupuis strongly recommended that Mitchell should go to The Johns Hopkins University for graduate instruction with the great mathematical astronomer Simon Newcomb. Arriving at Hopkins in 1895, Mitchell found that Professor New- comb had just retired as professor, and that Thomas Craig was head of mathematics and Charles Lane Poor of astronomy. He found Charles Lane Poor's teaching was of very high quality, and his per- sonality very attractive, so that Mitchell became inclined to adopt astronomy rather than mathematics as a life profession. He there- fore changed his plans, making astronomy his major subject, physics first minor, and mathematics second minor in working for his de- gree. Physics at Hopkins in those days took a very high place under Pro- fessor Henry A. Rowland. It used to be said, I know not how truly, that Professor Rowland was once asked in court "Who is the greatest physicist in the world?" "I am", said he. Being asked about it later, Professor Rowland said "I could not say otherwise. I was under oath." Professor Joseph Ames, a superb teacher, was Rowland's collabora- tor; L. E. Jewell was Rowland's assistant in preparing the great chart and table of the solar spectrum. The mechanic Schneider was Row- land's able aide in perfecting the dividing engine which ruled the famous Rowland gratings for spectroscopy. As he was one of ten children some financial help was essential. It was most gratifying, therefore, that an assistantship in astronomy was awarded Mitchell in his second year at Hopkins. His duties 256 BIOGRAPHICAL MEMOIRS consisted mainly in caring for the transit instrument and clocks in the little observatory back of the physics laboratory, and the 9%-inch refractor in the dome of the laboratory roof. In those years, when the fine Rowland diffraction gratings were becoming available, along with photographic dry plates of ever-in- creasing sensitiveness and spectral range, astronomic spectroscopy was flowering. Rowland's invention of the concave diffraction grat- ing opened new possibilities. His great and highly accurate map of the solar spectrum demanded equally accurate knowledge of the spectra of the chemical elements, in order to discover the constitu- tion of the sun. In Europe Kayser and Runge, Eder and Valenta, in England Sir Norman Lockyer and Alfred Fowler; in America Henry Crew and others, were gradually filling this gap by their laboratory studies of the arc and spark spectra of the elements. A few years be- fore, George E. Hale at Chicago, and Henri Deslandres in Paris, had independently conceived the device of the spectroheliograph for de- lineating areas of activity of single chemical elements on the sun. Hale had persuaded the rich merchant, Charles T. Yerkes, to pro- cure from Alvan Clark and Sons what is still the largest refracting telescope in the world, and to found the Yerkes Observatory, with Hale the Director. In these stimulating times, and being at the home of the Rowland gratings, Mitchell took as the subject of his doctoral thesis the use of the concave diffraction grating for photographing the spectra of the stars. Before that, stellar spectra had been taken exclusively with prisms mounted in slit spectrographs, with the slits fed by telescopes. At the start Mitchell also used a slit at the focus of the o^-inch tele- scope. But he was soon led to dispense with the slit and throw the star image directly into the focus of a concave grating. A special grat- ing of 6 inches aperture with lines 2 inches high was ruled on the Rowland dividing engine. Elected to Phi Beta Kappa in 1897, Mitch- ell was awarded the degree of doctor of philosophy in 1898, and his dissertation, including a theoretical discussion of the amount of astig- matism of a concave grating, used as described, was published in the Astrophysical Journal. SAMUEL ALFRED MITCHELL 257 Though he had received a fellowship at Hopkins in 1897 and it was continued by courtesy in 1898, Mitchell felt that it would be bet- ter to attempt to continue his research in astronomy, and he became a research student at Yerkes Observatory, which had been dedicated the year before. He says in an autobiographical paper written in his last years: "Looking back over my life, I believe the year spent at Yerkes was one of the finest years of my whole life, for I was able to imbibe a modicum of the enthusiasm for research found there, especially in my good friend of many years, Professor E. E. Barnard." He goes on to say: "The staff of the Yerkes Observatory, so soon after its opening, was very small. ... Its limited budget was supplemented from time to time by gifts from Mr. Hale himself, who actually was not in residence through the winter months. Professor Edwin B. Frost, who later became director of the Observatory, during the win- ter was in residence at Dartmouth College. The only senior as- tronomer was Professor Barnard, though Professor S. W. Burnham came up from Chicago on two nights of each week in order to make measures of double stars with the great 40-inch refractor. Ferdinand Ellerman, who worked closely with Mr. Hale for many years, and G. W. Ritchey who was then very busy in the observatory's basement grinding the two-foot reflector, were both in residence 1898-99. The winter temperatures were frightfully low. ... In the month of February 1899, the thermometer rarely got above zero Fahrenheit.... With the long nights of winter ... one could make a series of long exposures on stellar spectra. One night these exposures were continued for a period of 13 hours, while Professor Barnard was working at the same time at the 40-inch; and the temperatures in the domes ... fell to 260 below zero, Fahrenheit. "In those early days there were no warm clothes heated by elec- tricity. One could put on all the heavy flannels he owned and perhaps borrow a fur coat. One could keep comfortable for a few hours, depending on one's age and vitality, but gradually the cold would start at one's feet and creep up, so that before long one would be numb to the waist. ... It has always been a surprise that 258 BIOGRAPHICAL MEMOIRS none of the astronomers has been unfortunate to stub his toe when so numbed and weighted down with so much heavy clothing. A fall ... from the observing floor of the 40-inch when at its highest would have been catapulting a distance of about 60 feet." In a long voyage to observe the total eclipse of 1901 in Sumatra, and in later association at Mount Wilson Observatory, Professor Barnard has told the present writer how he used to keep awake through those long hours, night after night, with black coffee, while he spent many hours of the days between in photography and examining his beau- tiful star photographs. Mitchell's 6-inch concave grating in its box was attached to the 12-inch telescope, used as guiding instrument to bring the star ob- served to focus for the grating. With the small staff in residence, he was able to use the telescope as many nights per week as he wished. The spectrographs of stars he obtained gave interesting and promising results.