44 SECTIONAL ADDRESSES. 



In such attempts what Darwin called ' fool experiments ' and what 

 prospectors for oil call ' wild catting,' are not to be discouraged. Indeed, 

 many of the most fruitful discoveries are really made in this way. The 

 logic is put in afterwards. That is what happened in the case of the 

 3-electrode thermionic valve. 



Thanks to the work of Wright, Hubble and others, the source of excita- 

 tion in the bright line nebulse no longer appears inexplicable. We have 

 the cardinal fact that in nearly all cases stars of early type, capable of 

 affording radiations of high frequency, are involved in the nebulse. The 

 two or three apparent exceptions, though deserving of the closest scrutiny, 

 do not at present seem to have enough weight to upset a generalisation 

 which rests on a great number of cases. It is true that we cannot observe 

 these short waves, the maximum of intensity in the spectrum being 

 hidden from our view by the layer of ozone overhead, which I shall say 

 more of presently. But we can confidently infer their existence by 

 extrapolating from what we can see, and correcting for what we know of 

 atmospheric absorption. The cases of some of the central nuclei of the 

 planetary nebulse are specially satisfying from the definite relation 

 of the star to the nebula and the adequate character of the star itself. 

 W. H. Wright, writing in 1918, before these views had emerged, and 

 without any thesis to maintain, expressed himself as follows : — * 



' I cannot but believe that this wonderful richness in ultraviolet light 

 which gives the spectra of nebular nuclei their characteristic appearance, 

 in spite of the great difference which they exhibit in the matter of bright 

 bands, is the dominating peculiarity which must be regarded as the 

 distinguishing mark of this group of objects.' 



It has been suggested that the general penetrating cosmic radiation of 

 which we have heard so much of late stimulates the nebular spectrum ; 

 but upon the facts available this hypothesis hardly seems necessary or 

 helpful. 



The Dark Patches in the Nebulae. 



There is another aspect of the diffuse galactic nebulse which remains 

 obscure in more senses than one. It is seen to special advantage in such 

 objects as the ' trifid ' nebula in Sagittarius. Dark regions are, as it 

 were, interlarded with the bright ones in a way which strongly suggests 

 that we have to do with complementary aspects of the same phenomenon 

 somewhat in the same way that, for instance, the emission of a fluorescent 

 body is connected with its absorption. Yet it is very difficult to pursue 

 this line of thought into satisfactory detail. The opacity is quite 

 unrelated to the emission, and indeed it presents the baffling peculiarity 

 of having no peculiarity. For apparently every part of the spectrum 

 of the stars lying beyond is obscured in the same ratio. Experimenters 

 in the field of optics know how difficult it is to secure a result of this 

 kind in the laboratory, particularly when the ultraviolet spectrum has 

 to be included. 



Even fairly opaque gases like iodine vapour which are at our disposal 

 show markedly selective absorption, and in terrestrial experiment recourse 

 is usually had to the partial action of a solid obstruction such as a spinning 



1 Lick Observatory, vol. xiii, p. 252, 1918. 



I 



