387 



The banks of the river Trinidad are represented by the author as 

 being well suited for wharfs, especially in the neighbourhood of the 

 spot he recommends as the commencement of a rail-road ; but as the 

 mouth of the Chagres is impeded by a bar, he suggests the expe- 

 diency of forming a communication with the adjacent Bay of Limon, 

 which, in its present state, affords excellent anchorage, and which, 

 by making certain improvements in it, pointed out in the paper, 

 might, at a small expense, be rendered one of the most commodious 

 and safe harbours in the world. 



On the Law of the partial Polarization of Light by Reflexion. By 

 David Brewster, LL.D. F.R.S. L. & E. Read February, 4, 1830. 

 [Phil. Trans. 1830, p. 69.] 



When a beam of ordinary light is incident upon the surface of a 

 non-metallic body, at an angle having a certain relation with its re- 

 fractive density, the portion which is reflected is found to be com- 

 pletely polarized in the plane of reflexion ; but when the angle of 

 incidence is either greater or less than this, which is the polarizing 

 angle, the polarization is incomplete, as is proved by transmitting 

 the reflected beam through a doubly-refracting crystal, which de- 

 composes it into the ordinary and extraordinary rays. What a single 

 reflexion is unable to effect, may, however, be accomplished by a 

 sufficient number of successive reflexions at angles different from the 

 polarizing angle. The author had pointed out this fact in his com- 

 munications to the Royal Society in the year 1815 ; and the further 

 investigation of the law on which this phenomenon depends, has led 

 him to the discovery of the real change effected in light by its re- 

 flexion, and opened new views of that condition which constitutes 

 its polarization. The prevailing notion with regard to the condition 

 of the light which has been imperfectly polarized by reflexion, has 

 been, that it consisted of two portions ; the one wholly polarized in 

 the plane of reflexion, and the other not changed, but still retaining 

 the character of natural light. This doctrine was supported by Young, 

 Biot, Arago, and Fresnel, and more recently by Herschel. Dr. Brew- 

 ster contends, in opposition to these authorities, that every portion 

 of the reflected beam has suffered a physical change by the action of 

 the reflecting forces, some being completely polarized, others only 

 partially so ; complete polarization consisting in the effecting of such 

 a change in the position of the plane of polarization, as that they shall 

 be parallel to the plane of reflexion ; partial polarization, on the other 

 hand, consisting in these planes being only brought nearer to this 

 position of parallelism. In order to simplify the investigation, the 

 author begins by considering the case of a beam of light composed 

 of two polarized pencils, of which the respective planes of polarization 

 are at right angles to each other, which two pencils may be conceived 

 to be superposed upon each other. He then shows that the phe- 

 nomena exhibited by this compound pencil are exactly the same as 

 those exhibited by common or unpolarized light. He next proceeds 



2 c 2 



