16 



CONSTRUCTION OF TIIE MICROSCOPE. 



Dr. Arnott remarks : "But for this fact, which to many 

 persons might at first appear a subject of regret, as pre- 

 venting the distinct vision of objects through all trans- 

 parent media, light could have been of little utility to 

 man. There could have been neither lenses, as now ; nor 

 any optical instruments, as telescopes and microscopes, of 

 which lenses form a part ; nor even the eye itself." Rays 

 of light falling perpendicularly upon a surface of glass or 

 other transparent substance, pass through without being 

 beiit from the original line of their direction. Thus, if a 



Fig. 4. 



ray pass from Tc perpendicularly to the surface of the pieco 

 of glass at e (fig. 4), it will go on to h in the right line 

 k eo gh. But if the same ray be directed to the surface e 

 obliquely, as from a, instead of passing through in a direct 

 line to o in the direction aemb, it will be refracted to d, 

 in a direction approaching nearer to the perpendicular 

 line Jc h. The ray a e is termed the ray of incidence, or 

 the incident ray ; and the angle a e Jc which it makes with 

 the perpendicular k h is called the angle of incidence. 

 That part of the ray from e to d passing through the 

 transparent medium is called the ray of refraction, or the 

 refracted ray; and the angle deg which it make? with the 



