SECTIONAL ADDRESSES. 



SECTION A.— MATHEMATICAL AND PHYSICAL SCIENCES. 



THE ANALYSIS OF LINE SPECTRA. 



ADDRESS BY 



PROFESSOK A. FOWLER, F.R.S., 



PRESIDENT OF THE SECTION. 



Although spectroscopy formed the subject of Prof. McLennan's Presi- 

 dential Address to tlie Section so recently as 1923, I feel that no apology 

 is needed for returning to this subject on the present occasion. In the 

 three years which have elapsed spectroscopy has, in fact, made immense 

 progress in several directions. It has now become one of the most 

 important developments of physical research, and seems likely in the 

 near future to make contributions of a fundamental character to other 

 branches of science. Its development from a subordinate and little-known 

 adjunct of chemistry to the commanding position which it now occupies 

 forms a most interesting chapter in the history of science, and, as one who 

 has long been associated with spectroscopic research on the experimental 

 side, I may perhaps best approach the modern viewpoint by recalling some 

 of the more important stages in its progress. 



Rather more than sixty years ago, when the spectroscope became an 

 effective instrument of scientific research through the work of Kirchhoff 

 and Bunsen, it was regarded essentially as providing a new and powerful 

 method of chemical analysis. It soon had brilliant results to show in the 

 discovery of a number of new elements, but this kind of discovery could 

 not go on indefinitely, and the interest of chemists as a body in spectrum 

 aneJysis would appear to have declined rather rapidly. In contrast with 

 the present outlook in spectroscopy, it is interesting to recall that what 

 was then regarded as one of the greatest attributes of the spectroscopic 

 test was its extreme delicacy, so that Kirchhoff and Bunsen, for example, 

 were able to show that one three-millionth of a milligram of sodium could 

 be recognised with certainty. Spectrum analysis, however, as was soon 

 realised, was not so simple a matter as it first appeared, and called for so 

 much study that its pursuit was mainly left in the hands of a small band 

 of specialists. 



The introduction of the spectroscope into astronomy, which also 

 followed almost immediately the discovery by Kirchhoff in 1859 of the 

 nature of the dark lines of the solar spectrum, gave another interest to 

 spectroscopic investigations, which has continued to grow without a break. 

 Some of the most important developments of spectroscopy have, in fact, 

 been closely associated with attempts to interpret the spectra of celestial 

 bodies. It was not long before Huggins and Lockyer, who were prominent 

 among the pioneer workers in this field, realised that laboratory experi- 

 ments must go hand in hand with observations of the heavenly bodies. 



