198 PROGRESS IN MICROSCOPY 



ness e of the plates take effect. There may be some liquid between aoth 

 and the plates d and d' without this interfering with measurements. 

 The arrangement is set on the stage of a polarizing interference mic- 

 roscope. The Uquid area has the shape of a rectangle which is set 

 lengthwise at right angles to the fringes, whereupon the procedure 

 mentioned in § 3 (fringe-shift method) is apphed. Provided n' > n, 

 a flat wave, after passing through the micro-chamber, has the shape 

 shown in Fig. 6.2. The area BC in this figure corresponds to the 



t t t 



'■lb 



t=^ 



[V ..I I . VA . . 



-~-T 



t . \ t'M»ulrni*» II 



/ 



n 

 Fjg. 7.23. Micro-chamber for index measurements (liquids). 



area BC (index n) in Fig. 7.23. The flat areas on either side of BC 

 (Fig. 6.2) correspond to the surface-wave areas that travelled through 

 the plates d and d' of index n (Fig. 7.23). The aspect of the field is 

 shown in Fig. 7.18. The two images A[ and A'., are images of the 

 area BC (Fig. 7.23) where is the liquid /?. Let K be the fringe shift 

 expressed as fringe interspace, then (equation 7.9): 



{n'>n) n^n'-KXle. (7.11) 



Let us consider n' — n = 002, e =017 mm and Ae = 2 //. Measur- 

 ing K with an accuracy of approximately 1/40 of the space between 

 two fringes (A = 546 /^), n is correct within 2 to 3 units of four places 

 of decimals. Owing to temperature-originated index variations of the 

 liquid, the work is to be carried out at a steady temperature. As 

 mentioned in § 2, to preclude any error on A', a change-over to white 

 light is required. Therefore, dispersion of the liquid and the plates d 

 and d' should be not too different, d and d' may be substituted 

 (Fig. 7.23) for two plates whose faces B and C are sloping and not 

 perpendicular to the object-slide. Under such conditions, fringe de- 

 formation can be followed up in monochromatic light thus dispensing 

 with white light (Chapter VL § 1). Using a liquid whose index is known, 

 the fringe-shift direction is noted. Merely comparing this shift with 

 the one obtained from the undetermined liquid // shows whether n 

 is higher or lower than n' . 



