52 



LIGHTING AND FOCUSING 



[CH. II 



condenser may be lowered, and the concave mirror used. (See 

 also § 84.) 



REFRACTION AND COLOR IMAGES 



\ 96. Refraction Images are those mostly seen in studying microscopic 

 objects. They are the appearances produced by the refraction of the light on 

 entering and on leaving an object. They therefore depend (a) on the form of the 

 object, (b) on the relative refractive powers of object and mounting medium. 

 With such images the diaphragm should not be too large (see I S3). 



If the color and refractive index of the object were exactly like the mount- 

 ing 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 histol- 

 ogy. The air bubble in is 77 is an example of a purely refractive image. 



54- N> 55- N> 56. 



Figs. 54-56. 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 Hue N-N', the refracted ray by the bent line A C. 



\ 97. 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. 54 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 

 instead of C. 



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

 air it is bentfrow the normal, the ray C B passing to A and not in a straight line 

 to C". By comparing Figs. 55, 56 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 inci- 



