5° 



LIGHTING AND FOCUSING. 



\_CH. II. 



If the color and refractive index of the object were exactly like the mounting 

 medium it could not be seen. In most cases both refractive index and color differ 

 somewhat, there is then a combination of color and refraction images which is a 

 great advantage. This combination is generally taken advantage of in histology. 

 Fig. 89 is an example of a purely refractive image. 



53- N'. 54- N'. 55- 



Figs. 53-55. — Diagrams illustrating refraction in different media and at plane 

 and curved surfaces. In each case the denser medium is represented by line shad- 

 ing and the perpendicular or normal to the refracting surface is represented by the 

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



\ 93. Refraction. — Lying at the basis of microscopical optics is refraction, 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. 53, light passing from air into water 

 does not continue in a straight course but is bent toward the normal N-N', the 

 bending taking place at the point of contact of the air and water ; that is, the ray 

 of light AB entering the water at B is bent out of its course, extending to C in- 

 stead of to 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. 54, 55, 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. This relationship is expressed thus : Sine of the angle of incidence di- 

 vided by the sine of the angle of refraction equals the index of refraction. In 



the figures, 



Sin ABN 



-- index of refraction. Worked out completely in Fig. 39, 



Sin CB N' 



A B ^=40°, CBN = 28° 54' and -J^L^ = 0-64279 = r _ 33 ,-_ e the index of 



Sin 28° 54' 0.48327 

 refraction from air to water is 1.33. (See \ 30). In Figs. 54-55, illustrating refrac- 

 tion 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 getting the correct angle where the exact angle corresponding to the sine 

 cannot be found in the table it is necessary to proceed as follows : Find the sine in 



