SCIENCE PROGRESS 



response of these elements with regard to the stimulus existing 

 in the sun. 



Saha applies the same methods to the problem of tempera- 

 ture radiation in gases {Phil. Mag., 41, 267, 1921). The vapours 

 of some elements become luminous at moderate temperatures, 

 whilst permanent gases remain non-luminous at the highest 

 laboratory temperatures. The higher the ionisation-potential 

 of the element, the more difficult it becomes to cause it to 

 emit its line-spectrum. Now, the transition from a neutral 

 state to the ionised state is not abrupt, but proceeds through a 

 series of successive stages of equilibrium of the system with an 

 increasing number of quanta of angular momenta. It follows 

 that the radiation of normal lines will precede ionisation, and 

 that the temperature of emission of a certain group of lines 

 bears a definite relation to the temperature of ionisation. 

 The higher therefore the temperature of complete ionisation, 

 the higher will be the temperature of luminescence. In this 

 way — and by considering the second stage ionisation of the 

 elements — light is thrown upon the nature of spectral variation 

 with increasing age of a star, and the conclusion is reached 

 that the continuous variation of stellar spectral types can 

 mainly be ascribed to the varying value of the temperature 

 of emission of the stellar atmospheres. Thus the following 

 phenomena are related with the temperatures assigned and with 

 the spectral class stated : 



As far as the theory can be tested by observation, it appears 

 to be confirmed. Additional determinations of ionisation 

 potentials will enable it to be further tested. At present, it 

 may be regarded as a reasonable working hypothesis for ex- 

 plaining the differences between the Fraunhofer and flash spectra, 

 and the progressive change in the nature of stellar spectra 

 with increase in temperature. 



Stellar Parallaxes. — In Science Progress, 11, 97, 19 16, 

 an account was given of a method of determining the parallax 



