1880.] 071 the PJiotographic Spectra of the Stars. 295 



and more defined. The line at K is nearly as broad as H, and there 

 are other lines present. 



In the last spectrum of the map, that of Arcturus, we come to 

 that of a star of another order, which includes the solar type, but this 

 star appears to be further removed than the sun is, in the order of 

 change from the typical form, as we meet with it in Vega and Sirius. 

 Here the typical lines are no longer present as a strong gi-oup. The 

 line at K is stronger relatively to H than it is in the solar spectrum, i 

 The spectrum is crowded with fine lines, and in the visible part 

 resembles the solar spectrum, but beyond H the lines are more intense 

 and difterently grouped. 



We cannot resist the feeling that we have here to do with a star 

 which has departed farther from the condition in which Vega now is 

 than our sun has yet done. 



The question presents itself — Have we before us stars of per- 

 manently different orders, or have we to do with some of the life- 

 changes through which all stars pass ? 



Does the sun's position somewhere before Arcturus in the order 

 of change indicate also his relative age ? 



On these points we know nothing certainly. If I may give some 

 play to the scientific use of the imagination, I would ask you to 

 imagine an inhabitant from some remote part of the universe seeing 

 for the first time an old man with white hair and wrinkled brow, to 

 ask, Was he born thus ? The answer would be, No ; in this child, 

 this youth, this man of mature age, you see some of the life-changes 

 through which the old man has passed. So, giving play to the 

 scientific imagination, there may have been a time when a photograph 

 of the solar spectrum would have presented the typical lines only 

 which are still in Vega. At a subsequent period these would have 

 been narrower and more defined, and other lines would have made 

 their appearance. And if we allow this scientific imagination to 

 project these Friday evenings into the far future, the lecturer, clad 

 it may be in the skin of a white bear, may have to describe how the 

 spectrum of the then feeble sun has already passed into the class of 

 spectra which now distinguishes the stars which shine with red light. 



There remain only two other points. In 1865 I described the 

 method of observing the spectrum of a planet compai'ed directly with 

 the solar spectrum under similar conditions of terrestrial atmosphere. 

 The planet is observed in the early evening, when the light from the 

 sky is bright enough to give a spectrum. With a long slit one sees a 

 broad spectrum of the sky, and then upon it the brighter spectrum of 

 the planet. Making use of this method, spectra were taken of the 

 planets Venus, Mars, and Jupiter. 



I will now exhibit upon the screen the spectrum of Venus. This 

 broad spectrum is that of the light from the sky. The narrow 

 stronger spectrum is that of the planet Venus. You see line cor- 

 responds to line, and that there are no modifications or additions 

 which indicate a planetary atmosphere. 



