FRESNEL 209 



in Paris, who had been a soldier in Napoleon's army in 

 Egypt, but who was fond of scientific studies, find that such 

 polarised light may also be produced by ordinary reflection 

 at a glass or water surface. Reflection is likewise also a 

 means of recognition of the unsymmetry of such a polarised 

 ray of light. If, for example, such a ray is travelling verti- 

 cally, and can be reflected by a suitably inclined mirror in a 

 forward or backward direction, the reflection fails in the other 

 two directions, to right and left. From this it is directly 

 deducible, that such a ray is not similarly constituted in every 

 direction, one of the possible directions at right angles to it 

 being in some way preferred. If we are convinced, as by 

 Fresnel's mirror experiment (which also succeeds with 

 polarised light), that a ray of light is a train of waves, the 

 polarised ray cannot possibly be a train of waves vibrating in 

 the direction of propagation, as is the case with sound; for 

 if an alternating change of state of any kind existed in the 

 direction of propagation only, the properties of such a ray 

 would be similar in all directions at right angles to it. Hence 

 the polarised ray must oscillate transversely, and further, 

 when it is completely polarised, in only a single direction at 

 right angles to the ray. It still remained entirely unknown 

 what it is that changes periodically into its opposite in this 

 transverse direction, and in the other transverse direction at 

 right angles does not change at all. 



Fresnel's recognition of the nature of polarised light 

 became clearer, the further he penetrated into the study of its 

 properties. It is noteworthy that not a single one of the 

 most eminent members of the Paris Academy at that time - 

 neither Laplace nor Arago - was able to follow him. In- 

 deed, Fresnel himself appears to have had difficulties with 

 the idea of transverse oscillations; but he neverthless held 

 firmly to the actual signs given by nature, and followed them 

 - with complete success, as the future showed. The diffi- 

 culty lay in the fact that transverse waves only occur in solid 



Ps 



