210 



THEORY OF MICROSCOPIC OBSERVATION. 



Since with thin walls the reflexion is more complete and in many 

 cases total, the bright line becomes in this case also of greater 

 intensity. It is most brilliant in glass tubes filled with air and 

 immersed in water. 



The unequal refrangibility of the differently-coloured rays pro- 

 duces here, as with the air-bubble, narrow coloured fringes on 

 the bright line, the outer fringe appearing blue and the inner one 

 red, as is evident from the refractive indices. 



Another table may be serviceable here, exhibiting the influence 

 of the refractive index n on the distance of the bright line for 

 different values of r. The numbers have been partly determined 

 by interpolation, but are accurate to three places of decimals. 

 As unity we have taken the longer radius R, as above. 



We have, in the third place, to trace 

 the rays which enter the hollow space 

 and are reflected at the walls so that 

 they reach the objective only after a 

 four-fold refraction and a single re- 

 flexion. This course of the rays in- 

 volves, as in the corresponding case 

 of the air-bubble, the formation of a 

 second bright line, which always cor- 

 responds to the point in which the ray 

 emerging without deviation (if pro- 

 duced backwards) cuts the plane of 

 adjustment. For the determination of 

 this point let us take the ray S T 

 (Fig. 114) incident vertically from 

 below, whose angles of incidence and 

 refraction shall be designated by a and a', as before. The angles 



FIG. 114. 



