194 



NATURE 



[November 6, 1919 



adopted by several astronomers. The last volume, I 

 entitled "Inorganic Evolution" (1900), develops 

 the final methods and ideas of the author, and 

 presents a general classification of all the stars. 

 It is only necessary to add one remark : Sir 

 Norman is one of those who publish the observed 

 facts immediately, and also the interpretations 

 which present themselves at once to his mind. 

 This method inevitably involves imperfect detail, 

 or over-sanguine conclusions, which have been 

 freely criticised. Pruning and revision have 

 become necessary, and this work has recently 

 been taken in hand by the author himself. The 

 main body of facts and ideas remains unaffected, 

 and is always worthy of being retained. 



It will suffice to mention here very briefly on 

 one part some of the more important results on 

 the sun and the effects of its radiation, and, on 

 the other, the great classification of the stars. 



Sir Norman was the first to recognise the 

 presence in the solar spectrum of lines due to 

 a band spectrum, attributed at first to cyano- 

 gen, and now assigned to nitrogen alone. He 

 observed the widening of the dark lines in the 

 spectra of sun-spots, a phenomenon which has 

 since been so brilliantly explained by Prof. Hale, 

 of the Mount Wilson Observatory. 



With the simple arrangement of the objective 

 prism, he was the first to photograph in an 

 eclipse the spectrum of bright lines given by the 

 reversing layer, situated at the base of the 

 chromosphere, thus obtaining a verification of the 

 general accordance of these bright lines with the 

 ordinary dark lines, and confirming the simple 

 explanation of the dark lines given by Kirchhoff. 



He discovered in the fluctuations of the solar 

 prominences a period of 38 years, which is super- 

 posed on the great eleven-yearly period, and he 

 showed later, in collaboration with Dr. Lockyer, 

 that this same period of 38 years reveals itself in 

 variations of pressure of the terrestrial atmo- 

 sphere. This last result has a practical import- 

 ance because it renders possible the forecasting of 

 the variations of the monsoons in the Indian 

 Ocean. In addition, the schematic chart of the 

 law of the winds in the southern hemisphere, 

 drawn up in this case by Dr. Lockyer, has been 

 verified by all later observations; it has been 

 reannounced in 1919 by Prof. Hildebrandson, one 

 of the founders of meteorology, in a note on the 

 general movements of the atmosphere presented 

 to the Paris Academy of Sciences. 



One of the questions which have most occupied 

 Sir Norman is that of the variation of laboratory 

 spectra with the energy of the excitation. He 1 

 has from the first distinguished the long and short j 

 NO. 2610, VOL. 104] 



lines in the same spectrum, and the employment 

 of a very powerful induction spark has given him 

 new lines which he has called " enhanced lines.a|| 

 The three types of lines — long, short, and et^' 

 hanced — correspond with increasing temperature, 

 and constitute valuable tests which serve to dif- 

 ferentiate the stars. Sir Norman has observed 

 the presence of these lines in the spectra of stars, 

 and at the same time the different behaviour of 

 the lines of hydrogen, of helium, and of the 

 metals, which has led to a new classification of 

 the stars. The labour involved in this investiga- 

 tion was considerable, because it became neces- 

 sary to photograph stellar spectra under the un- 

 favourable conditions of London and with a high 

 dispersion. Its success was secured by the use of 

 an objective prism of large angle and by great 

 patience. 



At the same time, the great American astro- 

 nomer Pickering, with much more powerful 

 means, had entered upon the observation and 

 classification of stellar spectra over the entire 

 sky, and was content to use a small dispersion 

 which enabled him to reach the fainter stars. But 

 as the study of enhanced lines demanded a high 

 dispersion. Sir Norman confined himself to the 

 stars visible to the naked eye. |l| 



The classification adopted differs essentiall^" 

 from all previous classifications, which had con- 

 sidered only the actual temperatures of the stars 

 and supposed a continuous cooling. Sir Norman 

 went much further, and in the year 1888 estab- 

 lished a distinction between the stars in which the 

 temperature was rising, and those in which the 

 temperature was diminishing. Beginning with 

 a primitive nebula, the body which forms by con- 

 densation will at first become hotter, then attain 

 a stationary temperature, and will finally cool. 

 Its natural evolution, expressed by temperature- ^^^ 

 as a function of time, ought to comprise ^411^^1 

 ascending branch, a steady state correspond- 

 ing with the maximum, and a descending 

 branch. In the ascending phase the lines of 

 hydrogen are narrow and the chromosphere is of 

 low density ; at the time of maximum the en- 

 hanced lines predominate and the maximum in- 

 tensity of the spectrum is far in the ultra-violet; 

 in the later phase the lines of hydrogen are broad 

 and diffuse, and the chromosphere is of greater 

 density. It is certain that one thus penetrates 

 more deeply into the nature of things. Further, 

 Sir Norman does not explain the variable number 

 of metallic lines by a different distribution of the 

 chemical elements in the stellar atmosphere. 

 When the star is very hot the metallic lines are 

 wanting, and he has attributed this to a dissocia- 



