62 



LIGHTING AND FOCUSING 



[ CH. II 



images which is a great advantage. This combination is generally taken 

 advantage of in histology. 'The air bubble in \ 151 is an example of a purely 

 refractive image. 



\ 10S. Refraction.— Lying at the basis of microscopical optics is refrac- 

 tion, which is illustrated by the above figures. It means that light passing 

 from one medium to another is bent in its course. Thus in Fig. 61 light pass- 

 ing from air into water does not continue in a straight line but is bent toward 

 the normal N-N', the bending taking place at the point of contact of the air 



61. N' 62. N' 63. N' 



Figs. 61-63. Diagrams illustrating refraction in different media and at 

 plane and curved surfaces. In eaeh case the denser medium is represented by 

 line shading and the perpendicular or normal to the refracting surface is repre- 

 sented by the doited line N-N', the refracted ray by the bent line A C. 



and water ; that is, the ray of light A B entering the water at B is bent out of 

 its course, extending to C instead of C. 



Conversely, if the ray of light is passing from water into air, on reaching 

 the air it is bent from the normal, the ray C B passing to A and not in a 

 straight line to C". By comparing Figs. 62-63 in which the denser medium is 

 crown glass instead of water, the bending of the rays is seen to be greater as 

 crown glass is denser than 'water. 



It has been found by physicists that there is a constant relation between 

 the angle taken by the ray in the rarer medium and that taken by the ray in 

 the denser medium. The relationship is expressed thus : Sine of the angle of 

 incidence divided by tbe sine of the angle of refraction equals the index of re- 

 fraction. In the figures, ^ — t^wit?, = index of refraction. Worked out com- 

 oin Lrffv' 



pletely in Fig. 61, ABN=4o°, C£N'=2&° 54' and Sln 4 °° = °- 6 4^7 

 5 * ° 4 Sin 28 54' 0.48327 



1.33, i. e., the index of refraction from air to water is 1.33. (See § 39.) In 

 Figs. 62-63, illustrating refraction in crown glass, the angles being given, 

 the problem is easily solved as just illustrated. (For table of natural sines see 

 .third page of cover; for interpolation, J 38. ) 



\ 109. Absolute Index of Refraction. — This is the index of refraction ob- 



