')->-) 



PROGRESS IN MICROSCOPY 



E. Ingelstam and L. P. Johansson have investigated the point at 

 issue both theoretically and experimentally. When /; = 150 and 

 «' = 1, they found that a numerical aperture close to 0-25 provided 

 accuracy within //lOO if the object-originated path difference did not 

 exceed /. An aperture higher than 01 5 is not feasible if the same 

 accuracy is to be achieved with a 11 path difference. 



Therefore, if highly accurate measurements are required, the aperture 

 should be kept at values as low as image sharpness will allow. 



Two incompatible view-points stem from the foregoing when 

 accuracy in thickness and accuracy in width are both required 

 simultaneously (Ingelstam). If thickness accuracy (path difference) is 

 the paramount consideration, this can only be obtained at the expense 

 of transverse measurement accuracy, i.e. less definition. 



3. MEASURING ANGLES OF CONTACT BETWEEN LIQUID 

 AND SOLID SURFACES 



Formerly, when investigating the wetting of a solid, researchers 

 endeavoured to measure direct the contact angle, i.e. the angle formed 

 by the solid with the liquid surface. Accurate measurements were 

 difficult to secure with the instruments employed at the time and 

 have been replaced by more easily carried out dynamic measurements. 



/yj///,//^/J<.///////////y\, /V^-V/^ --/ 



I I \ ' 



Fig. 8.14. Drop of transparent liquid on a flat horizontal surface. 



The differential method, described in Chapter VIII, is so readily ap- 

 plied that contact-angle measurements are revived as they can be 

 carried out in the very small areas discernible in a microscope. Angle- 

 of-contact variations, originated from various causes, can thus be 

 gone into, step by step, with great accuracy. 



Figure 8.14 shows a drop of transparent liquid on a flat horizontal 

 surface and Fig. 8.15 the meniscus formed by contact with a vertical 

 solid wall. 



