16 



CONSTRUCTION OP THE MICROSCOrE. 



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." Eays 

 of light falling perpendicularly upon a surface of glass or 

 other transparent substance, pass through without being 

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



a 



Fig. 4. 



ray pass from h perpendicularly to the surface of the piece 

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

 k e g h. But if the same ray be directed to the surface e 

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

 line to h in the direction aemh, it will bo refracted to d, 

 in a direction approaching nearer to the perpendicular 

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

 the incident ray; and the angle aek 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 makes with the 



