l-Type doubling: Herzberg versus Nielsen

In 1942 Gerhard Herzberg demonstrated the existence of l-type doubling in a number of bending fundamental levels of linear molecules. He attributed the doubling partly to asymmetric-top effects and partly to Coriolis interactions with the stretching modes, but Harald Nielsen and Wave Shaffer claimed that it is a purely Coriolis effect. The dispute, which was due mainly to errors in theoretical calculations, lasted for several years. This paper presents a summary of the controversy, based on published papers and letters exchanged between the main participants.