172 Agronomical and Nautical Collections. 



pal sections are parallel, and becomes brightest when ihey 

 are perpendicuLir ; and then the extraordinary image va- 

 nishes. Thus each pencil is unequally divided, except in the 

 case when the sections make an ancrle of 45° with each other: 

 but when they are either parallel or perpendicular, each of 

 them will undergo a single refraction only, which is the same 

 with the former when the sections are parallel, but of a con- 

 trary nature when they are perpendicular to each other. 



It follows from these facts, that the two pencils, produced 

 by the double refraction, have not the same properties in va- 

 rious directions about their axes or lines of motion, since they 

 undergo sometimes ordinary and sometimes extraordinary re- 

 fraction, accordingly as the principal section of the second 

 crystal is directed parallelly or perjoendicularly to another 

 given plane. Supposing, then, that we draw right lines per- 

 pendicular to the rays in these planes, and conceive them to 

 be carried by the system of waves in its progress, they will 

 show the direction in which it exhibits opposite optical pro- 

 perties. 



The name of polarisation was given by Mains to this singu- 

 lar modification of light, according to a hypothesis which 

 Newton had imagined in order to explain the phenomenon : 

 this great mathematician having supposed that the particles 

 of light have two kinds of poles, or rather faces, enjoying 

 different physical properties : that in ordinary light the si- 

 milar faces of the different particles of light are turned in 

 every imaginable direction ; but that, by the action of the 

 crystal, some of them are turned in the direction of the prin- 

 cipal section, and the others in a direction perpendicular to 

 it, and that the kind of refraction, which the particles un- 

 dergo, depends on the direction in which their faces are 

 turned. It is obvious, that some of the facts may be ex- 

 plained according to this hypothesis. But without particu- 

 larly discussing it, and showing the difficulties, and even con- 

 tradictions to which it leads, when closely examined ; I shall 

 only observe, that the differences of the optical properties 

 exhibited by the two pencils, in directions at right angles 

 to each other, may also be comprehended by supposing 

 transverse motions in the undulations which would not be 



