4 



PROPERTIES OF LENSES. 



CHAPTER II. 



Properties of Lenses, Imperfections of Resulting 

 Images, Aberrations, Etc. 



According to the undulatory theory of light an ob- 

 ject becomes visible, when it communicates the rapid 

 vibratory motion of its molecules to the luminiferous 

 ether, which being propagated through it in the form 

 of spherical waves, is transmitted to the retina. 



The vibration of the molecules takes place at right 

 angles to the direction of the wave, for example, if a 

 long string of beads is shaken at one end, the vibra- 

 tions of the beads are at right angles to the length of 

 the string, while the waves propagated through it by 

 the shake move in the same direction as the length. 



Reflection. When a luminous ray falls, or is inci- 

 dent, on a polished surface, it is reflected at an angle 

 which equals its angle of incidence. (BAC and 

 B"AC Fig. 1.) 



Refraction. When a luminous ray passes from a 

 rarer to a denser medium it is bent out of its course. 



The Sines of the angles of incidence BAC, and re- 

 fraction B'AC, (Fig. 1), bear a constant ratio to each 

 other. 



This ratio is called the refractive index. 



Refractive Index. 

 Of Air 1-000294. 



Diamond 2*44 



>> 



Flint Glass 1*642 

 „ Crown Glass 1'550 



>> 



