ABSTRACT

To an instrument designer an anomaly is normally perceived as a nuisance, to a scientist it represents a challenge. It means that something is not properly understood, an interesting problem waiting to be solved. Gratings are a good example of this. Since 1902, when R. W. Wood [8.1] observed unexpected changes of diffraction efficiency by a factor greater than 10, and so sharply defined that it affected only one of the two sodium D-lines, we have followed his lead in calling this behaviour anomalous. Generations of spectroscopic scientists have investigated this phenomenon. There are several reasons for so much interest: