Jajid.uiy 1, 1900.] 



KNOWLEDGE 



11 



the ring which encloses our universe ; the true Mitgai-d 

 snake that encircles the entire world. Only to the 

 most constant and patient scrutiny will it give up its 

 secrets ; yet how large a proportion of the mystery of 

 our Cosmos is involved in an understanding of its 

 structure who can tell ? 



Thirdly, there is the Zodiacal Light. Wc in these 

 high northern latitudes are not well placed for watch- 

 ing it ; but it can be seen from time to time, and a 

 thorough use of the opportunities that do come will go far 

 to compensate for our less favourable position. And it 

 is worth mentioning, in this connexion, that the Gegen- 

 schein. the faint counterglow to the sun, more difficult 

 and elusive than the Zodiacal Light proper, was inde- 

 pendently discovered by an Englishman, and not a 

 dweller in Southern England at that, by Mr. Backhouse 

 of Sunderland. 



In the Zodiacal Light, and the Gegenschein, we have 

 again objects of the greatest interest and mystery, 

 which are quite unfitted for telescopic examination, are 

 truly naked-eye objects, and which to this day have 

 never been sufficiently observed. 



Fourthly, there are Aurora;. At the present period 

 of the sunspot cycle there is no reason to expect any 

 immediate recurrence of these beautiful phenomena. 

 But careful training in the knowledge of the constel- 

 lations and in the throe branches of work just men- 

 tioned will be the best possible preparation for properly 

 observing Aurorae when they set in again. And this is 

 most important. After a great display it is very easy 

 to collect a number of most vivid and jiicturesque de- 

 scriptions, but really useful and scientific accounts are 

 apt to be sadly wanting. 



All these four branches of astronomy are essentially 

 for the naked eye ; in a fifth, that of variable stars, 

 a great deal may be done without a telescope in the 

 strict sense of the word, that is to say, a good opera- 

 glass will suffice for a considerable number of objects. 

 An opera-glass also greatly adds to the number of 

 objects which are brought within the observer's range 

 of vision. In the series of papers to which the present 

 is intended to serve as introduction, it is my intention 

 therefore not to limit myself entirely to work which 

 can be done without any optical aid at all, but to in 

 elude in " Astronomy without a Telescope ' observa- 

 tions for which a good field-glass will suffice. 



My programme, therefore, may be divided into four 

 parts. First, lessons in the configuration of the con- 

 stellations, so that the principal stars may be easily 

 recognised. Second, simple observations with the naked 

 eye for training in the habits of astronomical work. 

 Third, observations with the naked eye of Meteors, the 

 Galaxy, the Zodiacal Light and Aurora. Lastly, obser- 

 vations with the help of an opera^glass ; mostly of 

 Variable Stars. 



THE CONSTITUENTS OF THE SUN. 



By A. Fowler, f.r.a.s. 



Of all the heavenly bodies open to our enquiries, 

 the Sun is the one which can be best submitted to the 

 processes of spectrum analysis ; in the first place 

 because its light is so brilliant that instruments of great 

 power can be utilised, and in the second place because 

 it is near enough to admit of its component parts being 

 separately observed. Through the use of spectroscopes 

 of high dispersion, and the increased attention given to 

 spectroscopic work during recent total eclipses, the data 

 at our disposal for deductions as to the chemical con- 



stituents of the sun have of late been enormously ex- 

 tended, and it may serve a useful purpose to briefly 

 summarise the present state of. our knowledge on this 

 subject. 



Information relating to the solar elements is arrived 

 at by three different routes. First of all, there is the 

 Fraunhofer spectrum of dark lines, by which we may 

 investigate the constituents of that part of the sun's 

 atmosphere which produces discontinuous absorption; 

 then there is the bright line spectrum of the chromo- 

 sphere and prominences ; and, finally, that of tho 

 corona. (Fig. 1.) 



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o- a 



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O o Cj 



o =.2 

 o g 



^ 3 O 



-5 i a 



2 i~ 



a c< „ 





■"■t H 



-^ U-, tt-i 



~ O O 



-= 5 3 oi 

 ^ ^ g S3 



Ml. " a 



° .B f^ 



As to the Fraunhofer spectrum, the most recent re- 

 search bearing upon the elements entering into the sun'i 

 composition is that of Professor Rowland, who has 

 catalogued close upon twenty thousand lines between 

 wave-lengths 2975.5 and 7331.2 by the use of his splen- 

 did concave gratings. Some hundreds of these dark 

 lines owe their origin to the absorbing powers of their 

 own atmosphere, through which the sun is of necessity 

 viewed ; but, as a rule, these are readily distinguished 

 from true solar lines by their increased thickness when 

 the sun is near the horizon, by their freedom from the 

 displacement which is common to all true solar lines 

 when the advancing or receding limb of the sun is 

 observed, or by their increased thickness when the air 

 contains a great deal of water vapour. (Fig. 2.) 



The chemical significance of tho true solar lines is 

 most satisfactorily determined by pliotographing side 

 by side the spectrum of the sun and that of the sub- 

 stance under investigation. Such a comparison at once 

 shows whether there are any coincidences of the solar 

 and terrestrial lines, and if there is an exact agreement, 

 we are entitled to conclude, in accordance with 

 Kirchoff's law, that the substance in question is present 



