18 BIOLOGICAL EFFECTS OF RADIATION 



the remainder fade away into a continuum. We infer that in such a case, 

 the excited states of the atom may be considered to form a theoretically 

 infinite sequence, such that the difference between the energy values of 

 any two consecutive states approaches zero as one traverses the sequence 

 in the direction of increasing energies. Actually there may be many such 

 sequences displayed by a single kind of atom, and the complexities of 

 even the simplest spectra are rather formidable. 



Since a spectrum line corresponds to a transition between two states 

 of an atom, it may be denoted by the symbols designating these two 

 states. Thus, the most prominent line (and the one most frequently 

 employed in research with ultra-violet light) in the ultra-violet spectrmn 

 of mercury — to wit, the line of wave-length 2537 A — is sometimes denoted 

 in this fashion: 



VS - 2'Po 



the symbols VS and 2^Po being applied by spectroscopists respectively 

 to the normal state and to a particular excited state of the mercury atom, 



o 



and the photons of wave-length 2537 A causing a transition from the 

 former state to the latter when they are absorbed. This is a most unwel- 

 come usage for anyone but a practiced spectroscopist, not only because 

 the symbols are intricate but because they are frequently changed, 

 generally in the direction of greater complexity: I recall at least five 

 alternatives for 2^Po. The biologist is not very likely to meet with this 

 usage, but if he does he had better appeal to some professional spectro- 

 scopist for a translation of these symbols into wave-lengths. 



Another difficulty which a biologist may, but is not likely to, encoun- 

 ter: a betterment of spectroscopic apparatus frequently leads to the 

 discovery that what has hitherto been regarded as a single line is actually 

 a cluster of lines very close together, i.e., differing very little in wave- 

 length. After such a discovery the cluster is still frequently described 

 as a single line, except in researches on the individual members of the 

 group. The aforesaid "line" of mercury is known to be a cluster of 

 at least five lines. 



CONTINUA IN ABSORPTION SPECTRA, AND IONIZATION BY LIGHT 



In the previous section I spoke of the continuum w'hich lies beyond 

 the highest distinctly separable lines of a series. This consists in part 

 of the uppermost lines of the series which are too close together to be 

 distinguished or "resolved"; but there is also a part of it which extends 

 beyond the limit of the series. The existence of this latter part is a 

 point of little practical but great theoretical importance, for when the gas 

 is irradiated by light of any wave-length comprised within this part, 

 the light expels electrons from the atoms: in this particular portion of the 



