Pancharatnam's route to the geometric phase

This article focuses on the route by which Pancharatnam was led to the geometric phase. Barring a near-miraculous result from spherical trigonometry which he pulls out of the air, it is seen to be a systematic attack on problems of analysis and synthesis of polarized light, entirely following from just one basic principle and presented as a series of propositions. It is also noted that the phase was quite central to the interpretation of this own experiments and to his other theoretical work