INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 107 



difference is (^-i-0-565// and, at A^r, d— 0-565 jn. The colours are no 

 longer the same. Let us take, for instance 6 =0015/<. The path 

 difference is 0580 /< at A'o and 0-550/^ at A'^. According to Newton's 

 scale of colours, the hue is indigo blue at Aq and dark red at A'^. 

 Hues are symmetrical in relation to the purple. Owing to the structure 

 of the phase-shifting object it follows that the colours are not the 

 same at every point of the images Aq and A^. The hues exhibit the 

 optical-path variations within the object and a cursory survey de- 

 termines accurately the phase variations. 



Altering the adjustment instrument will give zl = 0. The whole 

 field is dark, save in the areas Aq and A'^ which are bright. The dark 

 ground is the conventional one with the difference that the areas 

 A'q and A'e are coloured to a greater or lesser extent according to 

 the phase variations they give rise to, the colour showing directly 

 the phase-shift. Let us adjust the instrument so that J be very small, 

 004// for instance: the field becomes dark grey. The image is similar 

 to high-absorption phase-contrast. Here again, provided the object- 

 originated phase variations be adequate, they appear under the form 

 of colours. Otherwise, only variations in intensity are perceived. 



Increasing the phase-shift between the waves O and E, i.e. in- 

 creasing J, gives rise to coloured contrasts and, when J = 0-565 /li, 

 the purple colour is again visible. At higher J values, contrast is not 

 so good and sensitiveness drops. 



The best performance is achieved when J is very small and the 

 ground dark grey but then measurements are not so easily carried out. 

 The purple hue gives perhaps slightly less contrasted images but, 

 conversely, as shown later, measurements can be made easily. The 

 complete-image-duplication method just described is very sensitive 

 and very suitable for not too large objects: minute discrete details 

 can thus be observed under good conditions. As very large image 

 duplications are not feasible, the method is not applicable to very 

 wide objects as. then, the two images overlap. 



Type b microscopes. Differential method 



The following method applies to any object but is less sensitive. 

 Let us consider anew Fig. 3.12 and select a much thinner plate Lo 

 so that the lateral shift be small in relation to the object's width. In 

 the object's image A\ the waves O and E are arranged as shown in 

 Fig. 3.14. In imageless areas, the path difference between the two 

 waves equates A. In the area of the image A' itself the path difference 



