LNSTKUMENT CLASSIFICATION AND Al'l»LICATIO.\S 



image is transferred with a small magnifica- 

 tion to the image plane of a conventional 

 objective by means of a mirror system. This 

 instrument can be used with the highest 

 powers, but application to low powers is not 

 easy without restricting the field of view. 



An instrument due to Krug and Lau (10) 

 (Figure 8) uses an oblique splitting surface 

 below the objective with a reference surface 

 out to one side. This is suitable for low 

 powers and, in fact, cannot be used with 

 high powers because of the large space re- 

 quired by the oblique splitting surface. 



Several low-power instruments have been 

 described which form fringes between the 

 specimen surface and a partially reflecting 

 plate pressed into contact with it. By suit- 

 able selection of the reflectivity multiple- 

 beam fringes can be obtained, giving an 

 enhancement of sensitivity, but in such 

 instruments control of the fringe position is 

 diflacult. 



The instruments so far described in this 

 class are optically satisfactory, but the fact 

 that the mechanical connection between 

 specimen and reference surface is via the 

 microscope limb and slides makes the direct 

 measurement of path difference somewhat 

 difficult because of thermal drift and vibra- 

 tion. (The magnetic adjustment incorpo- 

 rated in the instrument shown in Figure 7 

 helps a little in this respect.) For this reason, 

 fringe measurements are better made from a 

 photograph. 



In many cases met with in engineering the 

 sensitivity of the interference method is too 

 high; thus, an ordinarily machined surface 

 gives a confused mass of fringes which is 

 difficult to interpret. A method was devised 

 by Zehender (11) to deal with this situation 

 A replica of the surface is made in a trans- 

 parent plastic, which is then examined by 

 transmission in a low-power interference 

 microscope. A suitable instrument is that 

 described by Dyson (12), in which a modified 

 Mach-Zehnder interferometer is built into a 

 microscope stage and very low powers are 



Holf- silvened 

 Sunfoce 



Fig. 8. Interference microscope of Krug and 

 Lau for opaque objects. 



used. By immersing the replica in a liciuid 

 of suitable refractive index, the sensitivity 

 can be lowered as far as may be desired. 

 Thus, using polymethyl methacrylate as the 

 replica material and water as the liquid, 

 each fringe corresponds to a height difference 

 of 0.00013" which is a suitable value for 

 engineering purposes. 



Instruments of Class II. An instrument 

 of this class, using an out-of-focus image of 

 the specimen as the reference field, would be 

 very promising, as it would be free from the 

 mechanical troubles mentioned above. How- 

 ever, reflecting objects usually introduce 

 large phase differences over their whole sur- 

 face and this would have an effect similar 

 to that of placing a piece of ground glass in 

 the reference beam at a point distant from 

 the focal plane, thereby very seriously re- 

 ducing the fringe contrast. 



It is for this reason that instruments of 

 Class II are conspicuous by their absence. 



Instruments of Class III. Because of 

 the fact that in this class of instrument the 

 path difference measured is that between two 

 points of the specimen separated by the dis- 

 tance of shear, the applications of such in- 

 struments are limited to the examination of 

 small areas of imperfection on otherwise 

 regular surfaces. An important class of such 

 observations is that of the determination of 

 the thickness of thin films by measurement 

 of the height of the step formed at the edge 

 of sach a film laid on a reflecting surface. 



417 



