RECENT ADVANCES IN SCIENCE 97 



the notion of the curvature of a curve in the plane or the 

 geodesic curvature of a curve on a surface. 



J. R. Conner {ibid. 29) uses beautiful and suggestive hyper- 

 spatial methods in a discussion of the rational sextic curve and 

 the Cayley symmetroid. 



ASTRONOMY. By H. Spencer Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



Stellar Spectroscopy. — An important series of investigations in 

 stellar spectroscopy by W. S. Adams of the Mount Wilson Solar 

 Observatory has recently been presented to the National 

 Academy of Sciences, Washington, and printed in their Pro- 

 ceedings, ii. pp. 143, 147, 152 and 157, March 191 6. To make 

 these investigations intelligible to the general reader it is 

 necessary to explain that stars have been classified by as- 

 tronomers into different types, according to certain spectral 

 characteristics ; this classification can, alternatively, be made 

 by measuring the colour of the star, using as a measure of the 

 colour the difference between the star's photographic and 

 visual brightnesses (called the colour-index). It is supposed 

 that the stars in their history undergo a process of gradual 

 evolution through successive types, but two theories have 

 been suggested as to the mode in which this evolution takes 

 place. The older and, until recently, generally accepted theory 

 was that a star commenced by condensing from a nebula as a 

 hot blue star which gradually cooled, passing through the 

 various types until ultimately it became a very red cool star 

 and finally a dark star. The newer theory supposes that a star 

 commences as a very diffuse mass of gas, with a spectrum of the 

 red type, which gradually condenses, getting hotter and bluer, 

 until a certain stage after which it gradually cools off again, 

 at the same time becoming redder. One of the most interesting 

 problems of astronomy to-day is to settle which of these theories 

 is correct. 



The first of the investigations referred to above provides 

 a new method for the determination of the type of a star, 

 which permits even of small variations in type being accurately 

 measured. Starting with a number of stars of known type, 

 estimates were made on a definite but arbitrary scale of the 

 differences in intensity between various lines in their spectra 

 due to hydrogen, and to calcium and iron ; it was found that 

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