345 



ON EIGHT. 



regard to (he polarisation of the direct rajt, their influence may' 

 be neglected. 

 Statement of This phenomenon in the last analysis may be explained in 

 geuaral^erms. *^^^ following manner : If a ray of light fall on a plate of glass, 

 and form wiih it an incidence of 35'^ 25', all the light which 

 it reflects is polarised in one direction or manner. And the 

 light which passes th.-Qugh the glass is composed^ first, of a . 

 quanljity of light polarised in a direction or manner contrary to 

 that which was reflected, and having a proportion to that quan- 

 tity, and secondly, of another portion not modified, but whicli 

 preserves the characters of direct light. 

 The rays re- These polarised rays have precisely all the properties of those 

 midfecaHoii"^ vvhicli are modified by the crystals which have double refraction j 

 as : y the doa- and, accordingly, what the author has said of them elsewhere 

 ble refraction. jr,^^y ^^ applied without restriction to the former. 



The author, by continuing his experiments on the polarisa- 

 tion of light, has observed the following facts : 

 Iceland soar ^ consider, says he, in order to fix the ideas, a vertical ray as 



used to distin- polarised with regard to the plane of the meildian, and J place 



^uish thecoa- beneath this ray a glass, not silvered, in such a manner, that it 

 oition of the j a ' •» ' 



transroitted can be turned round upon the ray so as constantly to make 

 ^^y^- with its direction an angle of 35^ 25', In order to analyse the 



light which is transmitted through this glass in its different po- 

 sitions, I place beneath it a rhomboid of Iceland spar, directing 

 its principal section in the plane of th'ri meridian. I shall call 

 the plane of incidence that which passes through the vertical 

 incident ray and the ray reflected by the glass. 

 Statement of The ray presents different phenomena according to the mo- 

 the effect as ^JQj^g qj. positions given to the glass itself. "When the glass has 



shewn by the , ^ ° , . . , r, ■ , 



chrystai. made a quarter of a revolution, it no longer reflects a single 



particle of light, and the ray that it transmits to the lower 

 crystal is refracted in the usual manner ; and, subsequently, the 

 reflected light diminishes, and the refracted light increases, from 

 the first position of the glass, until the plane of incidence has 

 described an arc of 90 degrees. The ray refracted in the ordi- 

 nary way by the rhomboid, also increases from the former unto 

 the latter position ; but the extraordinary ray only increases 

 until the plane of incidence has arrived at an angle of 45 de- 

 grees. It is then diminished, and becomes nothing when tl>e 

 class has performed one quarter of a revolution. Supposing, 



then. 



