Price 20 cents .EPRINT AND CIRCULAR SERIES OF THE NATIONAL RESEARCH COUNCIL INDEXING OF SCIENTIFIC ARTICLES Bv GORDON S. FULC; Corning Glass Works, Corning, New York Including an Analytic Index of the Astrophysical Journal, Vol. 51^54, 1920-1921 Announcement Concerning Publications of the National Research Council The Proceedings of the National Academy of Sciences has been designated as the official organ of the National Research Council for the publication of accounts of research, committee and other reports, and minutes. Subscription rate for the "Proceedings" is $5.00 per year. Business address: Home Secretary, National Academy of Sciences, Smithsonian Institution, Washington, D.C. The Bulletin of the National Research Council presents contributions from the National Research Council, other than proceedings, for which hitherto no appropriate agencies of publication have existed. The "Bulletin" is published at irregular intervals. The sub- scription price, postpaid, is $5.00 per volume of approximately 500 pages. Numbers of the "Bulletin" are sold separately at prices based upon the cost of manufacture. The Reprint and Circular Series of the National Research Council renders available for purchase, at prices dependent upon the cost of manufacture, papers published or printed by or for the National Research Council. Orders for the "Bulletin" or the "Reprints and Circulars" of the National Research Council, accompanied by remittance, should be addressed : Publication Office, National Research Council, 1701 Massachusetts Avenue, Washington, D.C. INDEXING OF SCIENTIFIC JOURNALS BY GORDON S. FULCHER The importance of the service which the subject-indexes of journals may render to scientific research is not generally realized. Yet, in order that the best methods may be chosen and unnecessary duplication avoided, each research should, of course, be based on as complete a knowledge of past results as may be obtained; and the task of guiding the scientist to those parts of the enormous accumulation of scientific literature which relate to his work natu- rally devolves largely upon the subject-indexes. Also, since the greater the amount of time and effort required by scientists to secure the information needed for effective research, the greater will be the proportion discouraged from attempting it and the greater will be the extent to which the research done will be either less efficient or more delayed than it should be, therefore the responsibility resting upon these indexes, particularly the indexes of abstracting and listing journals, to perform this important task well, is very great. The question then arises, What characteristics must the indexes have if they are to give the maximum service ? The indexes now provided are of various types and degrees of usefulness. The simplest is a mere unclassified index of titles, which are arranged alphabetically by their significant words. The indexes of the Astrophysical Journal, the Physical Review, and many other journals are of this type. An improved type is the classified index of titles in which the titles are arranged under a limited number of subject headings so as to bring related titles together, the classification being based, however, wholly on the titles. The indexes of Science Abstracts, Journal de Physique, and others are of this type. The inadequacy of both these types is obvious for it is well known that in most instances a title cannot sufficiently describe all the subjects treated in the article. A paper on "The Atomic Weight of Iodine" may contain results relating to I 2 O S and to the occlusion of oxygen by glass; and one on ''The Flora of Formosa" may describe new species and perhaps new genera. A much more serviceable type is the index of titles classified by content of the articles, which, while retaining the titles, recognizes their shortcomings and classifies each with more or less precision under headings determined by an examination of the article itself or an abstract. Thus "The Flora of Formosa" would be listed under the genera, species, and other subjects concerning which new information is given. The International Catalogue of Scientific Literature and the cumulated indexes of the H. W. Wilson Com- pany Agricultural Index, Industrial Arts Index, etc. are of this character. The value of this type of index is enhanced if, as in the case of the Engineering Index and the card indexes of the Concilium Bibliographicum, the classified titles are supplemented by brief statements as to the contents. Since, however, most titles are incomplete and many inaccurate, why not disregard them altogether? This is done by the content index. In preparing it, the precise subjects dealt with in each article are determined by an analysis of the original or of a reliable abstract, index entries which adequately describe these contents are formulated, and finally these entries are indexed under the proper subject-headings, alphabetically arranged. The index of Chemical Abstracts is of this type, being based on an analysis of the abstracts, and is not only one of the largest but also one of the most generally useful indexes now being issued. For the most part, however, the classification is carried through one stage only, with the result that if information on a certain phase of one of the larger subjects is desired, many references may have to be looked up. Moreover, the index depends on abstracts which vary con- siderably in their standards. The final step in the development of the subject-index is to base the entries directly or indirectly through abstracts on a thorough analysis of the original articles, and to carry the subclassification of the entries through two or three stages so as to make each sub- division complete. Since the articles in the Astro physical Journal for 1920 and 1921 were carefully analyzed in preparing the analytic abstracts 1 which precede the articles, it was possible to prepare 1 Gordon S. Fulcher, ''Scientific abstracting," Science, 54, 291-95, 1921. from them a very thoroughly classified analytic index. This is reprinted at the end of the paper. To show the characteristics of the analytic index as compared with those of two types of title index, the entries used by each type to index the same four articles, all relating to photography (Astrophysical Journal, 52, 86; 52, 98; 52, 201; and 53, 349), are collected here: Index of the Astrophysical Journal (Unclassified Title Index} Contraction and Distortion on Photographic Plates, Image Image Contraction and Distortion on Photographic Plates Images, Mutual Action of Adjacent Photographic Photographic Images, Mutual Action of Adjacent Photographic Plates, Image Contraction and Distortion on Photographic Sharpness and Resolving Power Photometry and the Purkinje Effect, Photographic Purkinje effect, Photographic Photometry and the Resolving Power, Photographic Sharpness and Index of Science A bstracts (Classified Title Index] Photography Images, Mutual Action of Adjacent Photographic 1 Photometry, Photographic, and Purkinje-Effect Plates, Photographic, Image Contraction and Distortion on Resolving Power, Photographic Sharpness and 1 Vision Purkinje-effect, and Photographic Photometry Analytic Index 2 Photographic plates; properties contrast functions drying, time of grain size images contraction and distortion mutual action sections 1 These references are put in by analogy with the others, as the 1921 index has not yet been issued. 2 Headings and subheadings alone are given; see the complete index herewith reprinted for details. intensification penetration of light resolving power sharpness shifts of spectrum lines and star images theoretical relations turbidity Photometry, photographic, heterochromatic Purkinje effect, photographic Spectra measurements errors possible shifts of lines photographic Stars measurements from photographs errors possible Sun spectrum shifts of lines photographic The precision and completeness of the analytic index as com- pared with the title indexes is evident. Moreover, since all the results relating to each subject are indexed together, the analytic index also provides a summary, in outline, of the work done in each field; that is, the reader interested in a particular subject, such as clusters, nebulae, spectra, variables, can determine in what phases of the subject progress has been made by consulting the proper sections of the index. On the other hand, the danger of indexing by titles alone is illustrated by the classification, in the Index of Science Abstracts, of the reference to the photographic Purkinje effect under "Vision'' whereas the article contains, in fact, no reference to the eye, the indexer being misled by the title. It may be objected that, while the analytic and other good content indexes are clearly more useful than any title indexes, they are too long and too difficult and laborious to prepare. But a comparison of the analytic index, here reprinted, with the corre- sponding four separate volume indexes of the Astro physical Journal shows that the analytic index has only twice as many words as the unclassified title indexes, though containing three times as many references, which are, moreover, precisely and thoroughly classi- fied. Also, the preparation does not require remarkable ability nor should it take an undue amount of the editor's time if the work is well organized. After deciding as to the classification to be adopted, the editor's task is merely to indicate on the abstracts the titles to be used to index the articles (heads and subheads being designated in some convenient way) and later to go carefully through the entries which have been typewritten on slips and arranged alphabetically. The clerical work can be greatly simpli- fied by the use of methods developed by such indexing agencies as Chemical Abstracts, the H. W. Wilson Company, and the New York Times. The subject-indexes now serving the various sciences vary widely in type and usefulness, as stated above. Some are very poor, some very good; but none is as complete and precise as it should be if it is to give maximum service to research in its field. It is only a question of time, however, when the workers in each science will come to realize the incalculable value of an efficient abstract journal combined with a complete, precise, and thoroughly classified subject-index, and they will arrange to make the small sacrifices of time and money necessary to secure such uniquely important tools. ANALYTIC INDEX OF ASTROPHYSICAL JOURNAL FOR 1920 AND 1921, VOLUMES 51-54, BASED ON THE ANALYTIC ABSTRACTS For references to spectra and spectroscopy of elements and compounds see Spectra. Absorption of light Atmosphere (see also Sky) by atmosphere, 315-290 MM; coefficients, 54, 297 absorption of light, 315-290 MM; coefficients, by vapors in King's electric furnace; Kirch- 54, 297 hoff's law applied to, 51, 13 absorption spectrum (Fraunhofer lines) (see Aethylamine, preparation of; method, 52, 129 Sun, spectrum) Angles, extremely small; interferometer method dispersion of light; measurement; stellar of measurement, 51, 257 interferometer method suggested, 51, 263 Arc ozone in; amount, daily variations, location, and low-current; anode fall, cathode fall, potential suggested origin, 54, 297 and potential gradient as functions of cur- Atomic theory, Bohr (see Spectra, theory) rent, to i amp., for various cathode ma- terials (Ag, C, Cd, Cu, Fe, Sn, Zn, and Benzene; dispersion, rotary magnetic, 436-620 MM; salts of Ba, Ca, Cs, Na, and Sr), 54, 273 54, 45 Benzene, nitro-; dispersion, rotary magnetic, 436-620 nn, 54, 45 Binaries, spectroscopic (see also Variables) Boss 373, elements of both orbits, parallax, and proper motion, 53, 201 Boss 3644, Virginis, elements of orbit, 54, 226 Boss 5026, elements of both orbits, 51, 187 Boss ssgi, elements of both orbits, parallax, and proper motion, 53, 201 +65 36g Camelopard, radial velocity, 52, 198 a Capella, elements determined by interfero- meter, 51, 263 49 8 Capricorni, elements of orbit, 54, 127 12, isK, 42, and 50 Cassiopeiae, radial velocities, 52, 198 19 Cephei, radial velocity variations, 51, 252 13 Ceti, binary component; elements of orbit, 52, no Class Oes: 19 Cephei, A Cygni, 9 Sagittae, and 4 more; radial velocity variations, 51, 232 A Cygni, radial velocity variations, 51, 252 X Cygni (Cepheid), spectroscopic orbit, 53, 95 65 r Cygni, binary component, elements of orbit, 53, 144 34 f, 68, 71, +62i637 Draconis, radial veloci- ties, 52, 198 205 Draconis, elements of both orbits, parallax and proper motion, 53, 201 dynamics; tides on sphere due to second sphere rotating around it, 51, 309 comp. a Hercules, elements of orbit, parallax and proper motion, 53, 201 Lalande 29330 and 46867, elements of orbit, parallax and proper motion, 53, 201 magnitudes, absolute (see parallax) measurement of relative brightness and position of components; interferometer method, 51. 257, 263 orbits; elements of, including periods, radial velocity curves, mass functions (see Boss 373, 3644, 5026, 559i; <* Capella, 49 6 Capricorni, 13 Ceti; X and 65 T Cygni, 205 Draconis, comp. a Hercules, Lalande 29330 and 46867, OS 82) measurement of; interferometer method, 51, 257 origin, theory of, from nebulae, 51, 309 7r Orionis, photometric study, 51, 218 02 82, elements of orbit, parallax, and proper motion, 53, 201 parallax and absolute magnitude (see Boss 373, 5591; 205 Draconis, comp. a Hercules, Lalande 29330 and 46867, O2 82, 53, 201) proper motion (see: Boss 373, 5591; 205 Dra- conis, comp. a Hercules, Lalande 29330 and 46867, 02 82, 53, 201) radial velocity (see orbits, also +65 "369, Camelopard; 12, isK, 42, and 50 Cassio- peiae, Class Oes; 34 ; computed from gold point to palladium point ratio, 51, 244 pressure on atoms and electrons; theory based on classical dynamics, 52, 65 between binary stars, 53, i on nebulae due to Galaxy, 53, i Radium emanation (Niton) occlusion in discharge tube, 54, 285 purification; modification of Duane's apparatus, 54, 285 Refractive indices (see Camphor, Limonene, Sugar, Tartaric acid) Satellites, diameter; measurement; interferometer method, 51, 257 Scandium, carbide; possible formation in electric furnace, 54, 28 Sky, night; brightness; various determinations; discussion, 52, 123 Solar corona and prominences (see Sun) Solar system, origin; tidal theory, 51, 309 Spectra and spectroscopy absorption spectra exploded wire, of Fe, X 2270-5645 A, spectro- gram, 51, 37 furnace, electric, of Ba, Ca, Co, Fe, Ni, and Ti; variation of relative intensities with temperature, 51, 13 production of electric furnace spectra, 51, 13 high temperature spectra, extreme ; ex- ploded wire method, 51, 37 theory (see halogen acid gases) Kirchhoff's law applied to electric furnace spectra, 51, 13 Zeeman effect, inverse (see Fe and V) air arc spectrum; two new lines, 54, 246 spark spectrum, condensed effect of self-inductance on relative intensi- ties, 590-872 MM, Si 236 identification of Ar, N, and lines, 590- 872 MM, Sit 236; 54, 76 shift with reference to vacuum tube lines, Sit 236 wave-lengths, 590-872 MM, Sit 2 36; 54, 76 ammonia bands, visible and ultra-violet; identification, 52, 301 arc spectra (see air, Fe, pole-effect, pressure shift) anode and cathode spectra; relative behavior of various metal lines; variation with atomic weight, 54, 65 comparison with furnace spectra (see Ca, cyanogen, and Swan bands) ionization lines; behavior, 54, 191, 246 low-current; variation with current; excita- tion stages for Ba, Ca, K, Mg, Na, Sr, 54, 191 for Ag, Bi, Cd, Cu, and Zn, 54, 246 relation of results to Bohr theory and Ritz equations, 54, 246 standard lines, secondary; comparison of i2-mm 5-amp. with 6-mm 6-amp. arc, 53, 260 argon; lines in condensed spark spectrum in air, 590-872 MM. 51, 2 36; 54, 76; shift with reference to vacuum tube lines, 51, 236 band spectra (see ammonia, cyanogen, halogen acids, Swan, water, C, CO, CO,, CF 4 , HaS, N, SO,) arc and furnace spectra; comparison of in- tensity distribution; CN and Swan bands, 53, 161 Deslandres' law; test with nitrogen positive bands, 52, 301 theory of structure of infra-red bands (see halogen acids) vacuum tube discharge through CO., HiS, NH 4 , N,O, N,O,, and SO,; bands excited in visible and ultra-violet, 52, 301 barium absorption spectrum, electric furnace; varia- tion with temperature, 51, 13 arc spectrum, low current; variation with current, 54, 191 classification of lines furnace excitation, infra-red, 51, 179 low-current arc excitation, 54, 191 infra-red furnace spectrum to 856 MM, at vari- ous temperatures, 51, 179 series of single lines and triplets; identifica- tion of terms; constants, 51, 23 binaries (see Binaries) bismuth arc spectrum, low current; variation with current, 54, 246 classification of lines, arc excitation, 54, 246 structure of XX 4122, 4308, 4722; Plate, 53, 323, 339 of XX 3397, 3511, 3596; Plate only, 53, 323 of X4722; changes in relative intensity of components, 53, 339 cadmium arc spectrum, low current; variation with current, 54, 246 classification of lines, arc excitation, 54, 246 new resonance line, X 3779, 54, 246 calcium absorption spectrum, electric furnace, 51, 13 arc spectrum, low current; variation with current, 54, 191 classification of lines furnace excitation, infra-red, 51, 179 low-current arc excitation, 54, 191 furnace spectra; absorption, emission and mixed, 51, 13; infra-red, 51, 179 infra-red furnace spectrum to 733 MM, at various temperatures. 51, 179 new lines, fifty, 52, 265 pressure shifts to i atm., 315-650 MM, 53, 224 series of singlets and triplets; identification of terms; constants, 52, 265 carbon bands, negative; origin, 52, 301 band, positive, fourth; wave-lengths, 52, 301 ultra-violet vacuum spark spectrum; X 1931- 360 A; Plate, 52, 47; wave-lengths, 53, 150 X-ray spectrum, L-series; identification, 52, 47 Spectra (con/.) carbon dioxide; bands in visible and ultra- violet, including several new, 52, 301 carbon monoxide; bands in visible and ultra- violet; identification, 52, 301 carbon tetrafluoride; bands in visible and ultra-violet; wave-lengths, 54, 133 cathode rays; intensity of N bands excited, as function of energy, 52, 278 classification of lines enhanced lines, Fowler's; discussion, 54, 246 furnace excitation stages, King's electric; for Ba, Ca, Co, Xi, and Sr; infra-red, 51, 179 for Mn, X 2795-6500 A, 53, 133 for Sc, X 3015-6559 A, 54, 29 low-current arc excitation stages for Ag, Bi, Cd, Cu, and Zn, 54, 246 for Ba, Ca, K, Mg, Na, and Sr, 54, 191 comparison with furnace classification, 54, 246 cobalt absorption spectrum, electric furnace; varia- tion with temperature, 51, 13 classification of lines; furnace stages; infra- red to 809 MM, 5 1 . 179 infra-red furnace spectrum to 809 MM, at various temperatures, 51, 179 copper arc spectrum, low current; variation with current, 54, 246 classification of lines; low-current arc stages and comparison with furnace stages, 54, 246 continuous background obtained with ex- ploded wire source, 51, 37 cyanogen bands intensity distribution, in furnace and arc spectra, 53, 161 X 3883 in arc and furnace; Plate, 53, 161 excitation of spectra; minimum voltage; he- lium spectra, 52, i; variation with current density; helium, 52, i exploded wire spectra (extremely high tempera- ture absorption and emission spectra) of Cu, Xi, Mn; continuous background, 51, 37 of Fe; absorption spectrum, X 2270-5645 A; Plate, 51, 37 production, method, 51, 37 use for study of pressure shift suggested, 51, 37 fluorescence spectra (see mercury, 54, 149) fluorine; spark spectrum, visible and ultra- violet, of pure gas; wave-lengths, 54, 133 Fraunhofer lines (see Sun, spectrum) furnace spectra, electric absorption spectra of metallic vapors comparison with emission spectra, 51, 13 production, method, 51, 13 theory"; Kirchhoff's law applied to, 51, 13 variation with temperature; spectra of Ba, Ca, Co, Fe, Xi, and Ti, 51, 13 Ba, Ca, Co, Xi, Sr; infra-red, 51, 179 Mn, 280-820 MM, 53. 133 Sc, 301-656 MM, 54 28 comparison with arc spectra intensity distribution in cyanogen and Swan bands, 53, 161 spectra of Ba, Ca, Co, Xi, Sr, 51, 179; Sc, 54, 28 comparison with solar spectrum; Sc, 54, 28 effect of small potential gradient, 52, 187 infra-red absorption spectra to 920 MM, of Ba, Ca, Co, Xi, and Sr, 51, 179 mixed absorption and emission spectra; production, method, 51, 13 origin of radiation; discussion, 52, 187 red fringe; explanation, 52, 187 variation with temperature (see furnace absorp- tion spectra) Zeeman effect for iron lines, 51, 107 grating spectrograph comparison with interferometer, 53, 260 ghosts and reversals; use of, in accurate measurements, 53, 260 intensity effect on wave-length nil, 53, 260 polarizing effect on reflected and transmitted light, 51, 129 ultra-violet, extreme, 52, 47, 286; 53, 150 halogen acid gases (HBr, HO, HF); bands HC1 band 3.7 M; wave-lengths, law of spacing, evidence of satellites, 53, 300 theory of structure of infra-red bands isotopic theory of doublets, 52, 248 quantum theory, based on simple molecular model of Bohr type, 51, 230 helium excitation of various spectra; minimum voltage, 52, i intensity, relative, of series lines and bands in arc spectrum; variation with voltage, 52, i ultra-violet, extreme, spark spectrum; identi- fication of lines, 52, 47 hydrogen, Balmer series shift of Ha; condensed spark in air com- pared with vacuum spectrum, 51, 236 variations in relative intensity of lines in spectrum of Class Md variable star, 53, 185 hydrogen bromide, chloride, and fluoride (see halogen acid gases) hydrogen sulfide; spectrum of discharge through 52, 301 infra-red spectra (see air, Ba, Ca, Co, Xi, Sr, halogen acid gases, sun, water) elimination of scattered light of shorter wave-lengths, 53, 121 screen for light to 7200 A, 53, 121 interferometer spectrograph comparison with grating, 53, 260 ghosts and reversals; use of, for accurate measurements, 53, 260 intensity effect on wave-length nil, 53, 260 reduction of measurements, method, 53, 260 ionization lines, in low-current arc; variation with current, 54, 191 Spectra (con/.) iron absorption spectrum furnace; variation with temperature, 51, 13 exploded wire, X 2270-5645 A; Plate, 51, 37 arc lines, \337o-67soA; wave-lengths of 1026 lines, measured with grating and interferometer, and compared with Bureau of Standards results, 53, 260 intensity effect for arc lines nil, 53, 260 pole-effect in Pfund arc comparison of i2-mm s-amp. with 6-mm 6-amp. arc, 53, 260 relation to Zeeman effect nil, 53, 320 ultra-violet spark spectrum to 200 A; Plate, 52, 47; wave-lengths, 53, 150 variations in relative intensity of lines in spectrum of Class Md variable stars, 53, 185 Zeeman effect furnace lines; direct and inverse effect for 100 lines; camparison with results for spark lines; Plates, 51, 107 relation to pole-effect nil, 53, 329 isotopes, components due to measurement of separation; displaceable slit-method suggested, 53, 329 theory, for case of bands of HBr, HC1, 52, 248 magnesium arc spectrum, low-current; variation with current, 54, 191 classification of lines; low-current arc stages, 54, ioi manganese classification of lines, 280-650 MM; furnace stages, 53, 133 furnace spectrum, 280-820 MM; variation with temperature, 53, 133 measurements errors, possible, due to mutual influence of adjacent photographic images, 52, 98; 53, 349 of shifts, minute; displaceable slit method, suggested, 53, 329 of ultra-violet wave-lengths, extremely short, 53, 150 mercury; fluorescence spectrum, excitation of; active molecules; relation to exciting spectrum, 54, 149 mixed absorption and emission spectra; pro- duction in electric furnace, 51, 13 nebulae; spectra (ses Xebulae) neon; low-voltage spectrum of trace of neon in helium, 52, r nickel absorption spectrum, electric furnace; varia- tion with temperature, 51, 13 classification of lines, furnace stages; infra- red to 780 MM. 51, 179 infra-red furnace spectrum to 920 MM; at various temperature?, 51, 179 ultra-violet spark spectrum to 731 A; Plate, 52, 47; wave-lengths, 53, 150 nitrogen; band spectrum excitation by vacuum discharge through N 3 O and NiOi, 52, 301 intensity as function of energy of the exciting cathode rays, 52, 278 new ultra-violet positive bands from low- current arc in air, possible, 54, 246 structure; divergence from Deslandres' law, 52, 301 wave-lengths of third positive, 52, 301 nitrogen; line spectrum identification of lines in condensed spark in air, 590-870 MM. 5i 236; 54 76 relative intensity; effect of self-inductance with spark, 51, 236 shift of spark lines with reference to vacuum tube lines, 51, 236 nitrogen peroxide; spectrum of discharge through, 52, 301 nitrous oxide; spectrum of discharge through, 52, 301 novae; spectrum (see Novae) oxygen identification of lines in spark in air and Oa, 590-870 MM, 51, 236; 54, ?6 shift of spark lines with reference to vacuum tube lines, 51, 236 pole-effect in iron arc, comparison of 6-mm 6-amp. and i2-mm 5-amp. arcs, 53, 260 measurement, displaceable slit-method sug- gested, 53, 329 relation to Zeeman effect, for iron, nil, 53, 329 potassium arc spectrum, low-cunent; variation with current, 54, 191 classification of lines, low-current arc stages, 54, IQI pressure shift calcium arc lines, 315-650 MM, 53, 224 source for study; exploded wire suggested, Si, 37 in stellar spectra, Arcturus, Procyon, 53, 327 quantum theory (see theory) radium emanation (niton) new lines, 398-745 MM, 54, 285 relative intensity of lines; variation during discharge, 54, 285 resonance lines in low-current arc spectra intensity variation with current for Ag, Bi, Cd, Cu. and 7,n, 54, 246 Ba, Ca, K, Mg, Xa, and Sr, 54, 191 Ritz equations (sec series) scandium classification of lines, furnace stages, 54, 28 furnace spectrum, 301-656 MM; at various temperatures; comparison with arc, solar and sun-spot spectra, 54, 28 Spectra (cont.) Swan band scandium (cont.) intensity distribution; 5165 A; arc and Zeeman effect for lines in sun-spot spectrum, furnace spectra, 53, 161 54, 28 theory series Bohr; relation of low-current arc results to, in Ba spectrum, singlets and triplets, 51, 23 54, 246 in Ca spectrum, singlets and triplets, 52, relation of minimum voltage results for 265 helium to, 52, i notation; explanation, 51, 23 isotopic, of separation of doublets of HBr and Ritz equations; relation of low-current arc HC1, 52, 248 results to, 54, 246 quantum, of structure of band spectra of shifts of lines (see spark spectra, Sun) halogens, based on simple molecular model measurement; displaceable slit method sug- of Bohr type, 51, 230 gested, 53, 329 titanium; absorption spectrum, electric furnace, photographic, due to mutual action of adjacent at various temperatures, 51, 13 images, 52, 98; 53, 349 ultra-violet spectra (see Ca, CO, CO*, F, Mn, X, silver NI j 4j NlO , N.O., SO,, Sun) arc spectrum, low current; variation with extreme, to 200 A (see C, He, Fe, Xi, Zn) current, 54, 246 measurements of wave-lengths, 53, 150 classification of lines; low-current arc stages, source; condensed vacuum spark, 52, 286 54> 2 -^ spectrograph, vacuum, 52, 47, 286; 53, 150 screens for region 290-315 MM, 54> 2Q7 arc spectrum, low current; variation with spectrograph, special double, for solar spec- current, 54, 191 trum, 290-315 MM, 54, 297 classification of lines; low-current arc stages vacuum discharge spectra (see CO, CO=, CF<, 54. iQi X, Xe, XH 4 , RaEm, SO,) Zeeman effect for D-lines; explanation of intensity of X bands as function of energy of Woltjer's observations, 51, 107 cathode rays, 52, 278 sources of light (see arc, exploded wire, spark, minimum voltage for excitation of He spectra, vacuum) 52, i exploded wire; appearance and mechanical spectra of discharge through CO*, HiS, XH 4 , effects of explosion, 51, 37 XaO, XjOa, SOj, 52, 301 spark spectra (see air, argon, fluorine, hydro- vacuum sources gen, nitrogen, oxygen) cathode rays from treated carbon cathode, relative intensity; effect of self-inductance on used to heat anode, 53, 323 air and O lines, 51, 236 condensed spark, 52, 286 shift with reference to vacuum tube lines; air, vanadium; Zeeman effect, direct and inverse, Ar, H, X, and O lines, 51, 236 for 90 furnace lines; comparison with ultra-violet, extreme (see C, Fe, He, Xi, Zn, effect for spark lines, 51, 107 ultra-violet) Venus; spectrum (see Venus) spark, condensed, in vacuum; as source for water vapor; absorption band, 930-963 MM, extreme ultra-violet, 52, 286 53, 121 spectrographs (see grating, interferometer) X-ray spectra standards, international secondary; questioned L-series of carbon; identification, 52, 47 lines ; comparison of 6-mm 6-amp. and Zeeman effect (see Fe. Xa, Sc, V) i2-mm 5-amp. arcs; pole-effect, 53, 260 furnace lines compared with spark lines, of stellar spectra (see Stars) Fe and V, 51, 107 strontium relation to pole-effect for Fe lines nil, 53, 329 arc-spectrum, low-current; variation with zinc current, 54, 191 arc spectrum, low current; variation with classification of lines current, 54, 246 arc stages, low-current, 54, 191 classification of lines, low-current arc stages, furnace stages, infra-red, 51, 179 54, 2 4& infra-red furnace spectrum to 920 MM; at ultra-violet spark spectrum to 316 A; Plates, various temperatures, 51, 179 52, 47, 286; wave-lengths, 52, 286 structure (see bands, bismuth) Stars sulfur dioxide bands atmospheres, pressure in; Arcturus and Procyon, in visible and ultra-violet, including forty 53, 327 new; wave-lengths, 52, 301 binaries (see Binaries and Variables) sun (sec Sun) brightness (see parallax) Stars (cont.) catalogue (see Cluster stars) in Pleiades; magnitudes and colors of 821 stars in region 2 square, 54, 323 spectroscopic parallaxes, magnitudes, type, and proper motion of 1646 stars, 53, 13 Cepheids (see Variables) Class Bo-Bs; statistical study of 180 stars; mean magnitude, parallax and proper motion, 54, 140 Class O; collision theory of origin, 54, 229 Class Oes (see Binaries) Class Md (see Variables) cluster (see Cluster stars) colors (see Cluster stars) determination for nebulous stars, 52, 8 nebulous stars, 47, including p Ophiuchi, a and 22 Scorpii, 52, 8 in Pleiades; 753 dwarfs, 54, 323 comparison stars, for Messier 72; photographic magnitudes of 29, 52, 232 constant stars, from photometric studies Bond 624, 53, 317 i and a Cassiopeiae, 54, 81 Class B: ir< Orionis, , e, and /z Tauri, 51, 193 TT< Orionis, 51, 193, 218 , e, and ;t Tauri, 51, 193 / and TT Persei, 53, 105 radial velocity constant (see radial) density of stars in Gal. long. +32, lat. 20, 52, 73 in Galaxy; distribution, 52, 23 in space, as function of parallax and magni- tude, 52, 23 diameters measurement with stellar interferometer, 51, 257 (see Interferometer) a Orionis, 53, 249 disk (see diameter) darkening of limb (see Variables) distribution of luminosity; interferometer method of study, 53, 249 distribution in space (see density) number of each magnitude per unit volume, luminosity curve, 52, 23 double stars (see Binaries, visible) dwarf stars, in Pleiades (see catalogue) Galaxy (see Galaxy) giants (see Cluster stars, Variables) magnitudes, absolute (see parallax) magnitudes, photo-electric, of j3, S, I, w Persei, 53, 105 magnitudes, photographic and photo-visual catalogue, of 1646 stars, 53, 13 Class Bo-Bs, 180 stars, 54, 140 comparison stars for Messier 72, 29, photographic, 52, 232 for Nova Persei No. 2, 36, 52, 183 in Messier 3 and 68 (see Cluster stars) in Orion, eight; photo-visual, 53, 317 in Pleiades ; statistical study of 821, 54, 323 scales, photo-visual; comparison of Barnard's with Mount Wilson, 54, 323 measurements from photographs; possible errors due to contraction effect, 52, 98; and mutual action of images. 53, 349 nebulous stars colors of 47, including p Ophiuchi, a and 23 Scorpii, 52, 8 spectrum R Aquarii, nebulous lines; Plate, 53, 375 nebulosity around six stars, 52, 8 novae (see Novae) number of each absolute magnitude; luminosity curve, 52, 23 (see density) parallaxes and absolute magnitudes catalogue for 1646 stars; spectroscopic and trigonometric results. 53, 13 Class Bo-Bs stars; mean for 180, 54, 140 Boss 1517, 51, 254 determination; methods accuracy, relative; discussion, 54, 140 interferometer method suggested, 51, 237 spectroscopic; description, 53, 13 distribution of stars in space and number of each magnitude, 52, 23 relation of magnitude to space velocity; statistical study of 1330 stars, 54, 9 relation of parallax to apparent magnitude and proper motion, 52, 23 pressure in atmospheres (see atmospheres) proper motion catalogue, for 1646 stars, 53, 13 Class Bo-Bs, mean for 180 stars, 54, 140 large; two faint stars near MSI, 54, 237 measurement; interferometer method sug- gested, 51, 257 radial velocity (see velocity, space) binaries (see Binaries) 16, 19, +S92395, +83I04 Cephei, 52, 198 constant for S Ophiuchi, TJ Serpentis, ft Pegasi, 52, 3i7 +73835 Draconis, 52, 198 a Hercules and comp., 53, 201 25 8 Ursae Majoris, 23 5 Ursae Minoris, 52, 198 spectra (see nebulous stars) combined bright and dark line spectra; explanation, 51, 13 variable (see Variables) spectral type catalogue for 1646 stars, 53, 13 theory; effect of passage of a star near or through a nebula, 53, 169; 54, 229 origin of Class O stars, 54, 229 variable stars (see Variables) velocity; radial, tangential and space distribution; frequency of each velocity; statistical study of 1350 stars, 54, 9 relation to absolute magnitude, 54, 9 '4 Sugar, cane; aqueous solution dispersion, rotary, magnetic and natural, 436- 620 MM, 54, 116 refractive index, one to one solution, 436- 620 MM, 54, 116 Sun corona; May 29, 1919; photograph, 51, i eclipse; May 29, 1919; Smithsonian expedition; brief report, 51, i (see corona) meteorites falling into; light expected from; theoretical discussion, 51, 37 prominence; May 29, 1919; photograph, 51, i October 8, 1920; very high; stages of growth; photographs with Ca line, 53, 310 spectrum, including Fraunhofer lines comparison with arc and furnace spectra; scandium lines, 54, 28 energy distribution, corrected for atmospheric absorption, 315-290 AIM, 54, 297 infra-red, 890-990 up; identification and origin of lines; wave-lengths -of 563 lines, including 50 solar; also Plate, 53, 121 shifts of lines atmospheric refraction, 53, 380 center-arc, predicted by anomalous dis- persion theory, 54, 92 mutual influence of adjacent lines; data from limb-center comparisons, 54, 92 (see photographic) photographic effect of adjacent lines, 52, 98; 53. 349 theory, anomalous dispersion; of gravita- tional shift, limb-center shift, and mutual influence shift, 54, 92 spot spectra (see spots) theory (see shifts) ultra-violet (see energy distribution) map, photographic; 315-290 MM, 54> 297 spots spectrum comparison with arc and furnace spectra; scandium lines, 54, 28 Zeeman effect for scandium lines, 54, 28 theory; cooling of rising gases, 54, 293 theory (see spectrum , spots) Tartaric acid; aqueous solution dispersion, rotary, natural, and magnetic, 436-620 MM, 54. n6 refractive index, for one to one solution, 436- 620 MM, 54 IJ 6 Telescope objective; diffraction by; effect on image of disk and combination of disks, including June; mathematical theory, 51, 73 Temperature scale brightness ratio, gold point to palladium point, 5i. 244 palladium melting-point, 51, 244 Theory (see Binaries, Dynamics, Electron, Grat- ings, Gravitation, Interferometer, Nebulae, Novae, Origin, Photographic plates, Radia- tion pressure, Spectra, Stars, Sun, Variables Transits; observation; effect of diffraction by telescope objective; theory, 51, 73 Triple systems 65 T Cygni; orbit of spectroscopic binary com- ponent, 53, 144 13 Ceti; orbit of spectroscopic binary compo- nent and perturbations due to fainter visual component, 52, no light curve of variable binary component of X Tauri, 51, 193 orbits; of binary components (see 65 T Cygni, 13 Ceti, and X Tauri) perturbations due to third body (see 13 Ceti and X Tauri) X Tauri; photometric study of variable binary component; orbit, light curve, effect of third body nil, 51, 193 Variable nebulae (see Nebulae) Variable stars Algol; photometric study, elements of eclipsing system, light curve, color of satellite, 53, 105 R Aquarii; spectrum, intensities and displace- ments of lines and nebular lines (Plate), 53, 37S y Argus; spectrum; temporary, shifting, absorp- tion He lines, 52, 39 7 Camelopardalis; photometric study; light curve and elements, 54, 217 RS Canum Venaticorum; light curve, elements, computed parallax, 53, 99 / Carinae; periodic variations of wave-length and spectral type, 54, 161 i H. Cassiopeiae; photometric study; light curve, elements, darkening of limb, 54, 81 SX Cassiopeiae; light curve, elements, 53, 165 T Cephei; periodic spectrum changes, 53, 185 U Cephei; photometric study; light curve, elements, evidence of tidal evolution, 52, 145 Cepheids (see I Carinae, X Cygni, Messier 72) light curves of 26, in Messier 72, 52, 232 light range, small; possibility of Cepheids with; suggestion, 51, 62 orbit, spectroscopic, for X Cygni. 53, 95 origin; collision theory of, 54, 229 magnitude, mean, for 26 in M?2, 52, 232 periods; in M?2, 26 variables, 52, 232 relation to spectral type, 54, 161 spectral type; range of variation and relation to period, 54, 16 spectrum; periodic variations in wave- length and type; / Carinae, 54, 161 theory (see origin) general conclusions; mean atomic weight; ratio of mass to radius, 52, 73 relation of period to brightness, 52, 73 variation binary theory; discussion, 51, 62 condition of; ratio mass to radius, 52, 73 Variable stars (cant.) Class Md; spectrum; periodic changes in emission lines (see T Cephei, X Cygni, R Hydrae, R Leonis, X Ophiuchi, R Ser- pentis, S3. 185; R Aquarii, 53, 375) classification of long-period variables, 53, 179 cluster variables (see Cepheids) color variation of typical, 51, 49 colors and magnitudes of 28 in M68, 51, 49 magnitude, absolute, of typical, 51, 49 new; in Ma; 17 probable, 51, 140 in Ms3, 56, 72, and 75; positions of 80 shown on photographs, 52, 73 in M68; 28, mostly typical, 51, 49 colors (see cluster) satellite of Algol, 53, 105 X Cygni; spectroscopic orbit, 53, 95 X Cygni; periodic changes of spectrum, 53, 185 darkening of limb; i H. Cassiopeiae, 54, 81 205 Draconis, probable eclipsing variable, 53, 201 (see Binaries) eclipsing variables light curve and elements (see Algol, y Camelo- pardalis, RS Canum Venaticorum, SX and i H. Cassiopeiae, U Cephei, RT Lacer- tae, X Tauri) probable, 205 Draconis, 53, 201 ellipsoidal variable (see ITS Orionis) evolution, tidal, of U Cephei; evidence, 52, 145 giants (see Cepheids) R Hydrae; variations in spectrum, 53, 185 irregular variables; collision theory of origin, 54, 229 RT Lacertae; light curve and elements, 52, 257 R Leonis; variations in spectrum, 53, 185 light curves (see Cepheids, eclipsing, ellipsoidal, and long-period variables) long-period variables classification, 53, 169 light curves of 66; constants, 53, 169 spectrum of R Aquarii, 53, 375 magnitude (see Cepheids) magnitude, absolute (see cluster, parallax) Messier 3, 53, 56, 68, 72, 75; new variables (see cluster) Messier 72; light curves of 26 Cepheids, 52, 232 nebulous variable (see R Aquarii) new variables (see cluster variables) B.D. +8i?27; +8i?30,52, 145 orbits; elements (see eclipsing variables) spectroscopic; X Cygni, 53, 95 origin, of Cepheids and irregular variables; collision theory, 54, 229 X Ophiuchi; variations in spectrum, 53, 185 ITS Orionis; photometric study; light curve and elements, 51, 218 parallax and absolute magnitude, of RS Canum Venaticorum, 53, 99 periods (see light curves, Cepheids) photometric study (see Algol, y Camelopard, i H. Cassiopeiae, U Cephei, X Tauri) TT< Orionis, constant star, 51, 193, 218 R Serpentis; variations in spectrum, 53, 185 spectrum; periodic variations in emission lines (see R Aquarii, y Argus, Cepheids, Class Md) suspected variables B.D. +io?i77i, 52, 9 Bond 624 in Orion; photometric study; Hartwig's elements incorrect, 53, 317 205 Draconis, 53, 201 S Persei, 53, 105 X Tauri; photometric study; light curve and elements for binary, 51, 193 theory (see origin) tidal evolution; evidence of; U Cephei, 32, 145 Wolf-Rayet star (see y Argus) Venus; spectrum; systematic shifts of solar lines; variation with zenith distance; explanation; Plate, 53, 380 X-rays L-series of carbon; identification, 52, 47 source; condensed vacuum spark, 52, 47 Zeeman effect (see Spectra) PRINTED IN THE U.S.A. 16