290 Dr. William Huggins [Feb. 6, 



Dewar on having inaugurated so fruitful a method of spectroscopic 

 investigation. Instead of the usual optical arrangement of glass, I 

 have substituted a lens of quartz, and a prism of Iceland spar, similar 

 to that which I have used in my star work. 



I will now ask Mr. Cottrell to throw first upon the usual screen 

 the visible spectrum. Even now, when no glass is used, you see how 

 brilliant are the blue and violet parts of the spectrum. The part of 

 the spectrum we shall have to do with in the stars lies for the most 

 part beyond. Now, if this prepared screen be held beyond, you see 

 that the invisible energy is translated for us into characters which the 

 eye can read. In the crucible we have the vapours of calcium and 

 aluminium, and we now see, not merely the ultra-violet light, but 

 the bright lines of these substances in this part of the spectrum. 



I now proceed to the results which have come out of this work. 



In 1865 I exhibited on the screen several coloured drawings of 

 spectra taken from the observations of Dr. Miller and myself in 

 illustration of the different kinds of spectra which the stars present. 

 It is desirable that I pass three or four of them in review before I 

 exhibit the photographic spectra corresponding to them. 



The first diagram represents the spectrum of Sirius. The spectrum 

 of this star may be taken as typical of the stars which shine with white 

 light. Most of the photographs belong to this class of star. Very 

 early Dr. Miller and myself called attention to the distinctive charac- 

 teristics of the spectra of stars of this class. The great distinguishing 

 feature of their spectra consists of three or four very broad and intense 

 lines. By a method of direct comparison we found three of these lines 

 to coincide with lines of hydrogen. The first corresponds to C of the 

 solar spectrum, the second with solar F, and the third with a line of 

 hydrogen near G. This last line near G appears as the first strong 

 line of the photographic spectrum. There are, indeed, numerous very 

 fine lines also present, but these are so delicate as to be seen, fitfully 

 only, except under the most favourable conditions. We satisfied our- 

 selves of the double line of sodium at D, the least refrangible of the 

 magnesium group, and a line at E — a line of iron — and some others. 

 This class includes the largest number of bright stars. The spectra 

 of the different stars of this class are chiefly distinguished from each 

 other by the greater or less breadth and diffuseness of these lines 

 of hydrogen, and also by various degrees of strength and visibility of 

 the finer lines. 



I will now show you the spectrum of another class of stars of 

 which the light is tinged with yellow. This spectrum is that of the 

 star Aldebaran. This class includes our sun. In this star the lines 

 of hydrogen are reduced to about the proportion they possess in the 

 solar spectrum. The other lines of the spectrum are no longer fine 

 and difficult to see. Here we have in full the triple line of mag- 

 nesium. I now show the spectra of two stars of a different class 

 indeed, but in both cases the light is orange. I will not stop to describe 

 these spectra, but pass to one more class, the stars in the light of 



