MEASURING REFRACTION INDICES— UNPOLARIZED MICROSCOPES 177 



shift d'. For instance, when 6 = 3A-f A/3, Fig. 6.5 is still the same 

 although, this time, b' = //3 differs from d. In monochromatic light, 

 all fringes are identical and a shift equating a whole-number times ?. 

 cannot be observed for the figures are superimposed. 



To obviate this difficulty, phenomena are observed in white light, 

 this requiring that the dispersion of the object and its surrounding 

 medium be not too extensive. Now, the instrument is adjusted to 

 bring the white fringe, corresponding to zl = in the position (1), 

 shown in Fig. 6.5. (Refer to Newton's colour scale. Chapter VII, § 1.) 

 Next, actuation of the screw K(Fig. 3.4) displays the fringes seriatim 

 so as to bring the same fringe in the object. In order to make no error 

 on the integral part of the path difference d (3/ in the previous example) 

 all that is required is to evaluate the so-measured rotation of the 

 screw V as /-wave-length fringe intervals. Were the white fringe 

 perceived, in the field of view, both in and out of the object, all that 

 is needed is to evaluate the distance of these two fringes as mono- 



i\--o 



Fig. 6.6. The path difference J increases from left to right. 



chromatically-spaced fringes. The measurement of the path-difference 

 fractional part d, i.e. b' {aI3 in the foregoing example), is accurately 

 carried out in monochromatic light. To determine which of the two 

 indices, ;/ or n', is the higher, is evinced by merely watching the motion 

 of the white fringe J = during rotation of the screw V. Now the 



Fig. 6.7. The path difference J decreases from left to right. 



wave arrangement in a given position of the screw V defines the wave 

 arrangement, i.e. whether the path difference J increases from left 

 to right, when the reference wave is below the wave that passed 

 through the object, conversely, decreases (Fig. 6.7). This is ascertained 

 by observing an object whose index is known. Let us bring the white 



