There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy. Hamlet There exists a fairly large number of textbooks concerned with spectrochemical analysis. Most of them deal with practical applications and instrumental factors, and provide the reader with the knowledge indispensable for conducting analyses with the help of emission spectra. Practical knowledge and experience are indeed important requisites for success fully exploiting the spectrochemical method in the field of analytical chemistry. As the method is essentially empirical, it is, in principle, a simple one, provided that we succeed in exciting all samples in an identical manner; for then, relative intensities of spectral lines can serve as the 'weights' by which to measure amounts of elements. However, creating the required constancy of excitation conditions is hampered by the very nature of the sample, whose composition profoundly influences the excitation characteristics of the light source. Therefore, spectrochemists are inevitably engaged in all the processes that determine the radiation output of the light source for a given sample. Dealing, with this ensemble of processes, that is, with 'excitation' in the widest sense, is the object of this book (cf. § 1. 1). The reader will seek in vain for enumerations of practical rules that would tell him how to tackle a particular analysis problem.