318 the president's address. 



observer, and partly to the extreme care necessary while making 

 the comparison of the two spectra, in order to ascertain with 

 exactness whether the presence or absence of the vapour of any 

 particular terrestrial substance is indicated by corresponding 

 coincidences of lines in the two spectra. A careful examination 

 of a large number of photographs has almost conclusively shown 

 that the larger stars may be arranged in a connected series, the 

 various types passing from the class of white stars, through 

 those resembling the nature of our sun, to the stars which shine 

 with an orange or red light. It has been suggested by Dr. 

 Huggins that these different types of spectra may probably 

 indicate the relative ages of the stars, or, at least, their relative 

 temperatures. 



Padre Secchi, , the distinguished Italian astronomer, has dis- 

 cussed a large number of his own eye observations, classifying 

 the stellar spectra into three types : — 1. The white or bluish 

 stars, like Sirius and Vega ; 2. Bed or orange-tinted stars, as 

 Betelgeuse and Antares ; 3. Spectra of the type of the sun, 

 such as Capella and Arcturus. The spectrum of each of these 

 several types is crossed by numerous fine lines, and direct 

 measurement has shown that they mostly correspond with 

 distinctive lines in the solar spectrum. 



A few of the stars have been observed to give bright instead 

 of dark lines, from which it may be inferred that a large 

 portion of their light is emitted from glowing gas, chiefly 

 hydrogen. This is the case in most of the temporary stars which 

 have appeared suddenly from time to time. In May, 1866, and 

 again in November, 1876, two of these remarkable outbursts 

 took place, the first in Corona Borealis, and the second in 

 Cygnus, each being equal to a star of the second or third magni- 

 tude, though a few days previously to their discovery, they were 

 invisible to the naked eye. Their spectrum of bright lines of 

 hydrogen revealed them to us as worlds in a state of gaseous 

 incandescence, produced probably by large quantities of gas 

 evolved from them during some vast internal convulsion, of 

 which we became cognizant only after the lapse of hundreds of 

 years. May we not reasonably conclude that, at a distant age, 

 these and similar stellar outbursts were literally stars on fire ! 



