OPTICS. 



LESSON 31. 



Optics. 



Lu'minous, shining by its own light. 



Transpa'rent, admitting rays of light to pass through. 



Opaque', stopping the rays of light. 



Ze'nith, a point in the heavens directly over our heads, the pole of 

 the horizon. Na'dir is a point diametrically opposite to the 

 zenith, constituting the other pole of the horizon. 



Optics is the science which treats of light, and of the 

 instruments by which it is applied to useful purposes. It is 

 one of the most interesting branches of natural philosophy, 

 but not one of the easiest to understand ; it will be neces- 

 sary, therefore, that you give to it the whole of your attention. 



Light, when emanated from the sun, or any other lumi- 

 nous body, is projected forwards in straight lines in every 

 possible direction ; so that the luminous body is not only 

 the centre from whence all the rays proceed, but every point 

 of it may be considered as a centre which radiates light in 

 every direction. The particles of light are so extremely 

 minute, that although they are projected in difterent direc- 

 tions, and cross each other, yet they are never known to in- 

 terfere, and impede each other's course. It is still a disputed 

 point, however, whether light be a substance composed of 

 particles like other bodies. In some respects it is obedient 

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

 be independent of them : thus, though its course is guid- 

 ed by the laws of motion, it does not seem to be influenced by 

 the laws of gravity. It has never been discovered to have 

 weight, though a variety of interesting experiments have 

 been made with a view of ascertaining that point. Some 

 suppose tliat the rays of light, instead of being particles^ 

 consist of the undulations of an elastic medium, which fills 

 all space, and which produces the sensation of liglit to the 

 eye, just as tlie vibrations of the air produce X\\e sensation 

 of sound to the ear. Most of the plienomena may be ac- 

 counted for by either hypothesis, but that of their being par- 

 ticles applies more happily to some of the facts respecting 

 the modifications of light by refraction and reflection. 



When rays of light encounter an opaque body, part of 

 them are absorbed, and part are reflected, and rebound just 



