204 LIGHT SCIENCE FOR LEISURE HOURS. 



in the work, are unable to devote the whole of the day 

 or probably even a large portion of the day to 

 observation of the sun. But apart from this we must 

 take into account the occurrence of unfavourable ob- 

 serving weather, and Lockyer speaks of days seemingly 

 fine, when certain indications in the appearance of the 

 prominence-lines assure him that observation is useless. 

 Doubtless the experience of other observers resembles 

 his in this respect. But this is not all. During a 

 great part of the 24 hours the sun is not above the 

 horizon at any of the European or American observing 

 stations. And then, lastly, even when he is above the 

 horizon, solar outbursts of enormous importance might 

 take place without any possibility that terrestrial 

 observers could become cognisant of the fact ; simply 

 because any outbursts in the central parts of the face 

 turned towards the earth and of the half turned directly 

 away from the earth, could not produce prominence- 

 phenomena outside the solar limb. The spectroscope 

 gives us an account indeed of disturbances taking place 

 on the sun's face ; but the account can be by no means 

 so easily interpreted as in the case of prominences seen 

 in the ordinary manner. 



When we combine these considerations with the 

 circumstance that a solar eruption lasts but a few 

 minutes, and that the observer is unable to examine 

 more than one portion of the sun's limb at a time, so 

 that many important eruptions might occur even while 

 he was engaged in the most attentive observation, we 

 see that outbursts like the one witnessed by Professor 



