RUMFUKD SPECTROHELIOGRAJ'H. 148 



found on a lii<>li-lev('l K., photograph, but it did not a])j)ear on a Kj 

 photograph. This was confirmed by other exposures. 



The results given by this first i^hotograph have been borne out in 

 subsequent work. It is fovmd tiiat the hydrogen floeculi are in gen- 

 eral dark, though they are sometimes bright in disturbed regions, 

 usually in the neighborhood of sun spots.* 



CONCLUDING RP2MAKKS. 



In concluding, Ave may ])erhaps be })erniitted to speak of a few of 

 the numerous investigations which can be undertaken hy the student 

 of solar physics. If j^roper use is to be made of the numerous meth- 

 ods of research which are now available, a large number of investiga- 

 tors will be needed, working, if, possible, on some coo})erative plan, 

 at many stations widely separated in longitude. Even the adequate 

 use of the spectrolieliograph alone would l)e beyond the capacity of 

 any single institution, for when suitably designed this instrument will 

 furnish as many photograj^hs of the sun as there are elements present 

 in its atmosphere, and in addition to these many others Avhich repre- 

 sent the peculiarities of certain lines. For example, we have already 

 seen that it will be desirable to ascertain in what degree photographs 

 taken with enhanced lines differ from those taken with other lines of 

 the same element. AVith a large image of the sun important results 

 might be expected to follow from a study of photographs of sun spots 

 taken with the aid of the widened lines and with l)right lines or other 

 lines which are peculiar to the spot. In view of the constant changes 

 which are going on in the sun. a fcAV photographs made in any of 

 these Avays Avill not suffice. AYhat is Avanted are series continued 

 through at least one sun-spot period, in order to discover the laAvs 

 which govern the intensity and the distribution of the various gases 

 and metallic vapors. Furthermore, the great importance of eruptive 

 phenomena, their comparative rarity, and the brief time in Avhich all 

 their phases are exhibited, call for special preparations and metliods 

 of work. Spectroheliographs capable of talving several photographs 

 at once through different lines Avill be essential for any suitable study 

 (yf eruptiA'e phenomena. If a chain of observatories Avell distributed 

 in longitude could arrange their Avork so as to keep the sun almost 

 constantly under observation, many important eruptions Avliich are 

 noAv lost would be recorded. 



But it is by no means sufficient merely to take photographs of the 

 sun Avith the spectroheliogi-aph. In order to extend greatly the range 

 of the attack, and also to explain the sj3ectroheliograph results, 



« Many additional interesting details and conclusions regarding both calcium 

 and hydrogen floeculi are given in the original paper and should he consulted 

 by the special reader. 



