312 WELLS'S NATURAL PHILOSOPHY. 



Thus the globular bottles filled with colored liquid, in the window of a 

 drug-store, exhibit all the variety of moving scenery without, sach as car- 

 riages, carts, and people moving in diflerent directions: the upper half of 

 each bottle exhibiting all the images inverted, while the lower half exhibits 

 another set of them in the erect position. 



Convex mirrors are sometimes called dispersing mirrors, as all the rays of 

 light which fall upon them are reflected in a diverging direction. 



What is ca- 671. That department of the science of 

 toptrics? optics which treats of reflected light, is often 

 designated as Catoptpjcs. 



SECTIONII. 



REFRACTION OP LIGHT. 



What is meant Light traverscs a given transparent sub- 

 by the retrac- stancc, such as air, water, or o^lass, in a straight 



tion of light? . ' . ' , 



line, provided no reflection occurs and there is 

 no change of density in the composition of the medium ; 

 but when light passes obliquely from one medium to an- 

 other, or from one part of the same medium into another 

 part of a difterent density, it is bent from a straight line, 

 or refracted. 



What is a me- 672. A mcdium, in optics, is any substance, 

 dium in optics ? gQ^jj^ Hquid, or gaseous, through which light 

 can pass. 



A medium, in optics, is said to be dense or rare, according to its power of 

 refracting light, and not according to its specific gravity. Thus alcohol, olive 

 oil, oil of turpentine, and the like substances, although of less specific gravity 

 than water, have a greater refractive power ; they are, therefore, called denser 

 media than water. 



673. The fundamental laws which govern the refraction of light may be 

 stated as follows ; 



What laws srov- Whcu light passcs from one medium into 

 tion*/u-Mr' another, in a direction perpendicular to the 

 surface, it continues on in a straight line, with- 

 out altering its course. When light passes obliquely from 

 a rarer into a denser medium, it is refracted toward a 

 perpendicular to the surface, and this refraction is in- 

 creased or diminished in proportion as the rays fall more 

 or less obliquely upon the refracting surface. 



