34 PRESIDENTIAL ADDRESS SECTION A. 



tions in a region hitherto unexplored, and which, to this dav 

 (1897), remain associated in my memory with the profound awe 

 which I felt on looking for the first time at that which no eye of 

 man had seen, and which even the scientific imagination could not 

 foreshow. The attempt seemed almost hopeless; for not only 

 are the nebulae very faintly luminous — as Marius put it, 'like a 

 rushlight shining through a horn' — but their feeble shining cannot 

 be increased in brightness, as can be that of the stars, neither to 

 the eye nor in the spectroscope by any optic tube, however great. 

 The nature of these mysterious bodies was still an unread riddle. 

 .... On the evening of August 29th, 1864, I directed the tele- 

 scope for the first time to the planetary nebula in Draco. I looked 

 into the spectroscope, no spectrum such as I expected ! A single 

 bright line only ! .... A little closer looking showed two other 

 bright lines on the side towards the blue, all the three lines being 

 separated by intervals relatively dark. The riddle of the nebulae 

 was solved. The answer, which had come to us in the light itself, 

 read: Not an aggregation of stars, but a luminous gas." 



The "New Astronomy," therefore, had its infancy during the 

 life of the present generation ; its growth has been phenomenally 

 rapid, and at the present time no big telescope is planned without 

 provision being made for spectrograph^ researches. 



The work of the pioneers I have named has since been followed 

 up by other workers, too numerous to mention individually. The 

 spectra of all the terrestrial elements, under very diverse physical 

 conditions, are still under investigation with ever-increasing pre- 

 cision, and the exact comparison of their lines with those of the 

 stars and the sun is still going on. There are very many blanks 

 in our knowledge still to fill up. The sun and the stars still show 

 a host of lines in their spectra which have not been "fitted" to 

 corresponding lines produced in our laboratories, and it may be 

 that they never will be. 



The highest temperatures to which we can attain in our 

 experiments fall far short of those attained by the stars, and 

 terrestrial spectra undergo such remarkable changes in passing- 

 from low to high temperatures that the complete elucidation of 

 the spectra of all the elements we know, and all their spectral 

 modifications under varied conditions, will still occupy the atten- 

 tion of experimentalists for very many years. 



The stars and nebulae reveal the existence of substances which 

 have so far never been discovered terrestrially, and there is still 

 a large field unexplored, waiting for the chemist, the mineralogist, 

 and the spectroscopist to investigate. Even in the spectrum of 

 our own sun the gaps due to uninterpreted lines are very big ones, 

 and no one knows the fascination of the problem until he has by 

 experiment "fitted" terrestrially produced lines to "unknown" 

 lines in celestial spectra. 



Before the advent of spectroscopic methods the only move- 

 ments of the stars which could be detected by the astronomer were 

 those across the line of sight. The stars are so inconceivably 

 distant that it requires the most minute accuracy of measurement 

 to detect the motions at all. The "blink" method of comparing 



