REFRACTION. 



11 



rays parallel to the axis, is concentrated or converged to the 

 focus, and hence the illumination at the focus is greatly inten- 

 sified. If a beam of parallel rays fall upon the mirror at an 

 angle to the axis, it will also be converged to a focus, which, 

 however, does not lie on the axis BCG. 



Fig. 4 shows the direction of reflection in 

 this case. This is the condition that obtains 

 when the concave spherical mirror is used 

 on a microscope. It has the additional prop- 

 erty, as compared with a plane mirror, of 

 concentrating light at the same time that it 

 changes its direction, thus producing a 

 much stronger illumination of objects to be examined. 



REFRACTION. It was noted above that a light ray travels in 

 a straight line. This is true only when the medium remains 

 the same. Light passing from one medium into a different 

 medium is bent out of its course, still moving, however, in a 

 straight path in the second medium, but in a different direction 

 from that in the first. The bending of light rays is known as 

 Refraction, and the action of microscopes depends on this im- 

 portant property. 



Fig. 5 illustrates the principle of re- 

 fraction. CD is an incident ray of light 

 passing from air into glass at the surface 

 of separation ADB ; DP is the refracted 

 ray, NDS is a perpendicular to the sur- 

 face of the glass, NDC (~-a) is the angle 

 of incidence, PDS ( = &) is the angle of 

 refraction, and HDP is the angle through 

 which the ray has been deviated from its 

 original path, CDH. The angle of refrac- 

 tion, 6, for a given angle of incidence, a, in air, varies for dif- 

 ferent media, as glass, water, glycerin, etc., hence the amount 

 of deviation of the refracted ray from the original path is vari- 

 able for different media. A ray passing from air (which is 

 usually taken as the standard of reference) to a denser medium 

 is always bent toward the perpendicular NDS; that is, the 

 angle of incidence is greater than the angle of refraction. The 

 incident and refracted rays, and the perpendicular to the sur- 

 face, lie in the same plane. In any medium, for example glass, 

 the size of the angle of refraction varies with the angle of inci- 

 dence, which means that the direction a ray will take after 

 refraction depends on the slant with which the incident ray 

 meets the refracting medium. There is, however, a constant 

 relation between the angles of incidence and refraction which 



