5o 



LIGHTING AND FOCUSING. 



[CH. II. 



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- N>. 



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 line 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 A B 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 in- 

 stead 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 di- 

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



the figures, _ — '^ o Af/ = index of refraction. Worked out completely in Fig. 53, 



Sin CBN' 

 A B N = 4o°, CB N' =28° 54' and- 



Sin 40° 



0,64279 



= i-33. 



,, 1. <?., the index of 



0.48327 



In Figs. 54-55, illustrating refrac- 



Sin 28 54' 

 refraction from air to water is 1.33. (See \ 30). 



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 interpolation 

 see p. 18, 2 29a). 



2 93 a. Absolute Index of Refraction.— This is the index of refraction ob- 

 tained when the incident ray passes from a vacuum into a given medium. As the 

 index of the vacuum is taken as unity, the absolute index of any substance is al- 

 ways greater than unity. For many purposes, as for the purposes of this book, 

 air is treated as if it were a vacuum, and its index is called unity, but in reality 

 the index of refraction of air is about 3 ten-thousandths greater than unity. 

 Whenever the refractive index of a substance is given, the absolute index is 



