LIGHT. Ill 



other reflections or refractions. A ray of light once polaris- 

 ed in a certain plane continues so affected throughout its 

 whole subsequent course ; and at any indefinite distance from 

 the point where it originally underwent the change, the di- 

 rection of the plane will be the same, provided the media 

 through which it is transmitted be air, water-, or certain other 

 transparent substances which need not be enumerated. If, 

 however, the polarised ray, instead of passing through water, 

 be made to pass through oil of turpentine, the definite direc- 

 tion in which it is polarised will be found to be changed ; and 

 the change of direction will be greater according to the 

 length of the column of interposed liquid. Instead of being 

 an uniform plane, it will have a curvilinear direction, 

 similar to that which a strip of card would have if forced 

 along two opposite grooves of a rifle-barrel. This curious 

 effect is produced in different degrees by different media. 

 The direction also varies ; the rotation, as it is termed, being 

 sometimes to the right hand and sometimes to the left, accord- 

 ing to the peculiar molecular character of the medium through 

 which the polarised ray is transmitted. 



Light is, perhaps, that mode of force the reciprocal rela- 

 tions of which with the others have been the least traced 

 out. Until the discoveries of Niepce, Daguerre, and Talbot, 

 very little could be definitely predicated of the action of light 

 in producing other modes of force. Certain chemical com- 

 pounds, among which stand pre-eminent the salts of silver, 

 have the property of suffering decomposition when exposed 

 to light. If, for instance, recently formed chloride of silver 

 be submitted to luminous rays, a partial decomposition en- 

 sues ; the chlorine is separated and expelled by the action of 

 light, and the silver is precipitated. By this decomposition 

 the colour of the substance changes from white to blue. If 

 now, paper be impregnated with chloride of silver, which can 

 be done by a simple chemical process, then partially covered 

 with an opaque substance, a leaf for example, and exposed to 



