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22. — The Polakisation' of Light. 

 By James U. Harford. 

 [Abstract of Lecture delivered 12th October, 1876.] 

 Plate II. 



The subject of Polarisation of Light forms part of the general 

 one of Physical Optics. In attempting its special consideration, 

 therefore, it is difficult, if not impossible, to avoid reference to the 

 more extended and complete range of study with which it is con- 

 nected. An endeavour will, however, be made to confine the 

 attention as closely as possible to the proper and peculiar pheno- 

 mena of polarisation, and general references will be made only 

 where they may be found absolutely indispensable. 



Light must be understood, without further definition, to mean 

 that combiiiation or mutual relation that is known and acknow- 

 ledged to exist between the " luminous " or *' light-giving " body, 

 and the " sensation" of light for wliich provision is made in the 

 animal organisation. The external cause on the one hand and the 

 animal sensation on the other — light and vision — must both be 

 taken into account as completing our notion and idea of light- 

 Physical cause and human perception are, therefore, the simplest 

 data on which ground can be broken on this occasion. 



The first thing to be done now is to make a simple and clear 

 distinction or contrast between what we understand as ordinary 

 light and polarised light respectively. To all ordinary view the 

 two are alike, and it is only when some test or method of exami- 

 nation and analysis is applied that the difi'erence is discovered. 

 Ordinary light has been described, with fair propriety, as " all 

 sided," polarised light as " one sided." Let this be explained. 

 Ordinary light (Fig. 1 ) may be viewed in all directions — may be 

 transmitted through all transparent media in all directions of 

 position of the latter — may be reflected from all surfaces at what- 

 ever the angle of position of the reflecting surface, without any 

 apparent hindrance or alteration in its properties. Polarised light, 

 on the contrary, vdll not bear reflexion at a certain angle of the 

 reflecting surface, and will not bear transmission through a trans- 

 parent medium in certain positions of the two relatively. These 

 positions, on the one hand of complete transmission or reflexion, 

 and on the other of total obstruction of transmission or reflexion, 

 are found to be at right angles with each other (Fig. 2). At inter- 

 mediate positions or relations the transmission or reflexion are pro- 

 portionately partial. 



Thus polarised light possesses relations to two Hues of direction 

 at right angles to each other; and from this circumstance, in analogy 

 with the poles of the earth or spliere, light in the condition termed 

 "polarised" owes in part its distinctive appellation. The term 

 " in part" is used, because, when we come to consider some of the 

 phenomena in coimexion Avith crystallisation, there seems a further 

 reason for the appropriateness of the term. 



