70 Proceecliiigs of Philosophical Societies. [Jan. 



Brewster, containing further experiments on light, was read. The 

 paper was divided into five seelions. 1 . On the polarizing property 

 of the agate. When an agate is cut in a direction perpendicular to 

 the direction of its plates, it has the property of polarizing light. 

 Round the luminous object a nebulosity is seen which never disap- 

 pears completely. If a prism of Iceland crystal be interposed and 

 turned round, the nebulosity is found brightest when the luminous 

 object disappears, and faintest when the luminous object is brightest. 

 Hence Dr. Brewster concluded that the nebulosity was an imper- 

 fectly formed image of the luminous object, and that the agate 

 was imperfectly crystallized, and possessed a certain degree of 

 double refractive power. But upon attempting to separate the two 

 luminous images by means of agate prisms, he could not succeed. 



On Thursday, the 16th of December, Dr. Brewster's paper on 

 light was continued, 2. On the structure of the agate as connected 

 luith its action on light. The fibrous and foliated structure of the 

 agage^ and the alternate zones of transparent and opaque ])ortions, 

 were particularly explained, and the efi'ect which they produced on 

 the rays of light shown. 3. On the polarizing property of the 

 agate. Dr. Brewster had observed that on each side of the image 

 seen by means of the agate there was a highly coloured image. 

 This phenomenon is explained more in detail, and an attempt made 

 to connect it with the structure of the agate. 4. On the depola- 

 rizing property of bodies. Dr. Brewster liad formerly observed that 

 when a ray of liglit is polarized, if it be transmitted through mica 

 or topaz held in one particular direction, it is not altered ; but if 

 these bodies be placed in another direction, the light is depolarized. 

 The first of these directions he calls the neutral axis ; the second, 

 the depolarizing axis of these bodies. He has since observed that 

 almost all crystallized bodies possess this property of depolarizing 

 light. He found it likewise in horn, gum arabic, and in some 

 kinds of glass. He observed likewise that some crystals have the 

 property of polarizing and depolarizing light on the same body. 

 5, On the elliptical coloured rings exhibited by depolarized light. 



On Thursday, the 23d of December, the reading of Dr. Brew- 

 ster's paper on light was concluded. He described various curious 

 phenomena connected with the elliptical coloured rings ; but we 

 cannot pretend to make them intelligible without the assistance of 

 figures. The paper concluded with a general recapitulation of the 

 principal topics discussed in the essay, together with some other facts 

 stated elsewhere. Among these may be mentioned the following: 

 -^The liglit from the clouds and the sky is mostly polarized. The 

 rainbow consists of polarized light. Moon-light is not polarized. 



At the same meeting a paper by Anthony Carlisle, Esq. was 

 read, giving an account of a pecnliarity of structure in the human 

 body continued for four generations. This peculiarity exists in the 

 family of Zerah Colburn, the American boy, exhibited in London 

 about a year ago, and remarkable for his arithmetical powers. He 

 came from the feiate of Vermont, in North America. He had six 



