MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 187 



the colour of the image A[ then gives: 



n < n' (n'-n)e = 0-565- 0-332 = 0-233// . (7.6) 



Note 1. The first-order purple was selected as field colour sur- 

 rounding the images A[ and A', merely because it is readily detectable. 

 Naturally, all measurements can be resumed selecting another back- 

 ground colour and testing the data as often as required. Thus, in the 

 previous numerical example, if the microscope is adjusted for c = 0-332,w 

 the field surrounding the images is bright yellow instead of purple. 

 The left-hand image {A[) becomes lavender grey (a = 0097//) but the 

 difference 0-332 — 097 =0-235// remains the same. 



Note 2. The flat-tints process just described does not apply when 

 dispersions of the medium n (the object) and the medium n' (medium 

 encompassing the object) differ overmuch as tints depend on the 

 variations of the ratio (// — «') e/A in the visible spectrum. When 

 changing over from the radiation /"(O 480 //) to the radiation C (0-565,a) 

 the previous ratio takes on the values {np—n'p)elX and (nc—n'c)el?.. 

 Therefore, it is the difference between {np—n'p) and {tifj—rt'c) that takes 

 effect. Such difference, equating {np—nc) — {n'p—n'c) is the difference 

 between dispersions. If the tints are to remain the same as in Newton's 

 scale, the numerator of the ratio (/; — /?')e/A must not be a function of /.. 

 Consequently, n—n must remain constant, i.e. dispersions must be 

 the same. Otherwise the tints are no longer those in Newton's scale 

 and cannot be detected. 



2. MEASURING PATH DIFFERENCES, APPLYING THE FRINGES PROCESS 



(FULL-DUPLICATION PROCESS) 



Measurements may be carried out by substituting tint discrimination 

 for interference-fringe shifts. Such fringes can be originated in several 

 ways, but, regardless of the device employed, phenomena remain 

 the same. The following example should make it easier to comprehend 

 origination and application of these fringes. Let us revert to the 

 diagrammatic microscope shown in Fig. 3.19 and work in mono- 

 chromatic light. We have seen (Chapter III) that a slit, located in 

 the condenser focus (not shown in Fig. 3.19), should be used: it is 

 imaged in the focus of the objective Oj, where the Wollaston prism W 

 lies. The imaged slit is to be aligned with one of the Wollaston-located 

 fringes. Let us shift slightly the Wollaston W in the direction of the 

 microscope axis by moving it, for instance, away from the objective. 



