1 90 7-1908.] Nebulce and Nebular Hypotheses. 6g 



prism. The prism has the effect of impeding the passage 

 of the waves through it, and of throwing them out of their 

 straight course. Naturally the short waves are more affected 

 than the longer and steadier waves, and the consequence is 

 that when the beam of light emerges from the prism the 

 waves are no longer promiscuously mixed up in it, but are 

 arranged strictly in order of length — the longest waves at 

 one side of the beam, the shortest at the other, while waves 

 of intermediate length occupy intermediate positions. Hence 

 one side of the beam is now coloured red, the opposite side is 

 coloured violet, and between those two extremes we have 

 every possible shade, each passing by the finest gradation into 

 the next. Like a fairy wand, the prism has sorted out the 

 tangled skein, and arrayed the different constituents in orderly 

 sequence. The coloured band thus obtained is called a spec- 

 trum — the one I now show on the screen being known as a 

 continuous spectrum : light of every wave-length is involved 

 in its production, and there are no blanks in it. 



But under certain circumstances the spectrum is found to 

 be (^T^scontinuous : it is crossed transversely by a number of 

 lines or bands, and these may be dark or bright according to 

 the nature of the source of light. Now those lines are the 

 really important thing in spectrum-analysis, for it is known 

 that every element exhibits a perfectly definite set of lines 

 in its spectrum ; and no two substances have the same family 

 of lines. In other words, every substance is hall-marked by 

 its spectral lines, and is consequently always recognisable by 

 them. All our terrestrial substances have been thus exam- 

 ined, and the lines in their spectra carefully determined. 

 Thus when we pass the light of the sun through a spectro- 

 scope and examine the lines in its spectrum, we are able to 

 say at once whether any of our terrestrial substances exist 

 in the sun. The same argument applies to the light of a 

 star, and the result of such analysis is to demonstrate con- 

 clusively that matter is common property throughout our 

 universe, and that the sun and the stars are composed for 

 the most part of the very same materials as our earth. I 

 say for the most part, because exceptions do exist : gold, for 

 example, does not betray its presence in either sun or stars ; 

 while, on the contrary, certain substances exist in those far-off 



