INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



123 



X2X!, be quite parallel, the same phenomenon occurs at all points 

 of the plate. The path travelled by To is longer than that of 7^ ; the 

 difference is 2e{n = 1) assuming e to be the interferometer thickness 

 (distance from XiX[ to .Yo.To). Likewise, the path travelled by T3 is longer 

 than that of To, the difference still being 2e. Hence, every ray travels 

 along a path the difference of which is 2e in relation to the previous ray. 

 All the Ti, Ta, Tg, ... rays interfere and the light-intensity through the 

 plate is a function of the interferometer thickness e and of the wave 

 length / of the light used. Provided the same source (steady ?^) is 



Fig. 3.33. Multi-wave interferences. 



retained and e be varied, the light-intensity transmitted conforms to 

 the law shown in Fig. 3.34 in which the ordinates denote the inter- 

 ferometer-transmitted light-intensities as a function of the optical 

 thickness ne (n being the index of the medium between Xix[ and :>^2.^2)- 

 If the thickness ne is equivalent to A, 2A, 3A, etc., maximum hght is 

 obtained whereas, as soon as these values are diverged from, intensity 

 drops rapidly. The more reflective are the faces Xix[ and XgXo, the 

 sharper the peaks shown in Fig. 3.34. But, then, absorption of the 

 device increases, thus requiring rather intense monochromatic sources, 

 not readily obtainable. However, a satisfactory performance is evinced 

 by evaporating multiple layers on Xix[ and Xox',. Figure 3.34 shows 

 that, to obtain maximum light, the thickness must be accurately 

 determined as the slightest variation of e entails at once a considerable 

 intensity drop. This is the very fact made use of to evince transparent 

 objects. Let us consider the transparent object A, sandwiched between 

 the interferometer plates (Fig. 3.33). The muhiple rays passing through 

 the object A do not follow the same path as those passing in the 

 random area B close to said object. Phenomena occur in the area A 

 as if the spacing of the interferometer plates were altered. The light- 

 intensity varies at once, thus disclosing the presence of the object A. 



