1889.] of the various Species of Heavenly Bodies. 159 



VII. Binary Stars, Class 4. Yery Unequal Magnitudes, the smaller Star being 



Blue. 



VIII. Binary Stars, Class 5. Unequal Magnitudes, the fainter Star being Bed. 

 IX. Outstanding cases. 

 X. Conclusion. 



INTRODUCTION. 



In the Bakerian Lecture given last Session* I detailed the spectro- 

 scopic evidence which in my opinion shows that the various orders 

 of nebulae and stars are produced by the presence and subsequent 

 condensation of meteoric swarms in space, the most uncondensed ones 

 giving rise to the appearances which we term nebulae, the more con- 

 densed ones to those which we term stars. 



Since the lecture was delivered, my assistants and myself have been 

 employed not only in continuing the experiments, but in bringing 

 together and co-ordinating as great a number of recorded observa- 

 tions as possible, along those lines which seemed likely to furnish the 

 most severe tests as to the validity of the conclusions stated in my 

 former communications. 



Among the lines on which this work has been done are the follow- 

 ing: 



1. Spectra of Comets. Here the test is as follows : It is generally 

 accepted that comets are meteor-swarms in the solar system. They 

 get brighter, and therefore they must be hotter, as they approach the 

 sun. Their spectra, then, if my hypothesis is true, must resemble 

 those of gradually condensing swarms outside the system. 



2. Spectra of Aurora. Here the test is as follows : 400,000,000 

 meteorites, big and little, are encountered by the earth every day. 

 The air should contain some of their debris. If in auroras the solid 

 particles are acted on by an electric current, the spectral phenomena 

 presented by glow tubes should be reproduced to a greater or less 

 extent in the spectrum of the aurora. 



3. Origin of Double Stars. Here the test is as follows : The ap- 

 parently single variable stars of the Mira type are on the hypothesis 

 produced by the interaction of two or more swarms ; they are in fact 

 double nebulae. Visible physical doubles are probably then of the 

 same nature ; if so, in the present absence of complete knowledge of 

 their spectra, colour phenomena may help us to discuss their probable 

 origin. 



* See 'Roy. Soc. Proc.,' vol. 41-, p. 1. 



