INTRODUCTION TO OPTICS, 



SECTION I. On Optics. 



OPTICS is one of the most interesting branches of Natural Philosophy: 

 it is the science of vision, and teaches us how we see objects. In 

 this science, bodies are divided into luminous, opaque, and transparent. 

 A luminous body is one that shines by its own light as the sun, the fire, 

 a candle, &c. But all bodies that shine are not luminous: polished 

 metal, for instance, when it shines with so much brilliancy, is not a lumi- 

 nous body, for it would be dark if it did not receive light from a luminous 

 body : it belongs, therefore, to the class of opaque, or dark bodies, which 

 comprehend all such as are neither luminous nor will admit the light 

 to pass through them ; and transparent bodies are those which admit 

 the light to pass through them, 



such as glass and water. Trans- Fig. 1. 



parent or pellucid bodies are fre- 

 quently called mediums ; and the 

 rays of light which pass through 

 them are said to be transmitted by 

 them. Light, when emitted from 

 the sun, or any other luminous 

 body, is projected forwards in 

 straight lines in every possible di- 

 rection, or at least appears to move 

 as it would on that supposition : 

 so that the luminous body not only 

 seems the general centre whence 

 all the rays proceed, but every 

 point of it may be considered as a 

 centre which radiates light in every 

 direction (Jig. 1). A ray of light 

 is a single line of light projected 

 from a luminous body ; and a pen- 

 cil of rays is a collection of rays 

 proceeding from any one point of 

 a luminous body, as^g-. 2. 



Philosophers are not agreed as 

 to the nature of light. Some main- 

 tain the opinion that it is a body consisting of detached particles, which are 

 emitted by luminous bodies, in which case the particles of light must be 

 inconceivably minute, since, though they must cross each other in every 

 direction, they are never known to interfere with each other; others sup- 

 pose it to be produced like sound, by the undulations of a subtle fluid 

 diffused throughout all known space. In some respects, light is obedient 

 to the laws which govern bodies ; in others, it appears to be independent 



Fig. 2. 



