Biographical Memoirs :: MEGHNAD SAHA

As one of the interesting and immediate results following from the ionization theory, Saha not only was able to explain the absence in the solar spectrum of the lines of Rb and Cs because of the low ionization potential of these elements, but also to predict that their resonance lines were likely to be observed in the relatively cooler region of the sunspots. H. N. Russell, following Saha's prediction, looked for and found the infra-red pair of Rb-lines, but no Cs-lines, in the Mount Wilson photographs of the spot spectra which, incidentally, had been taken before the publication of Saha's theory.

We may recall at this place that, apart from J. Eggert's paper referred to earlier, a definite suggestion about high-temperature ionization, prior to Saha's papers, was also made by the Oxford Physicist F. A. Lindemann (later Lord Cherwell) in connexion with his controversy with S. Chapman about the origin of magnetic storms (Philosophical Magazine, December 1919). He gave the ionization formula for hydrogen, and discussed the possibility of the complete ionization of hydrogen in the solar chromosphere. Lindemann, however, did not further develop or generalize the formula; and above all he failed to notice that the theory of thermal ionization (with accurate values of ionization potentials obtained from spectroscopic data or otherwise) constitutes the key to the interpretation of stellar spectra with their almost bewildering complexity. It was Saha who first recognized this, and worked out the consequences in considerable detail, and he did this independently of Lindemann's suggestion about the high-temperature ionization of hydrogen.

In 1919 Saha was Awarded the Premchand Roychand Scholarship of the Calcutta University, and this made it possible for him to spend some two years in Europe. He first went to London and spent about five months in the laboratory of Professor A. Fowler. Later he moved to W. Nernst's laboratory in Berlin, and did some experimental work on the conductivity or heated caesium vapour to seek an experimental verification of the theory of thermal ionization. The results were inconclusive. Some years later, when at Allahabad, Saha again returned to this problem, and an account of the investigation, jointly with K. Majumdar and N. K. Sur, was published in the `Zeitschrift fur Physik' ( 1926) .