494 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS . ß '.. / :i• ": '. '"' :: J :" .2.•...:: :/':'7 • .... .•. Figure 7.--Beginning corrosion (reflected light phase contrast: 500X). actual relief depth. This makes reflected light phase contrast more sensi- tive by another factor of 2. The human eye can detect intensity difference of around 5%. With this approximate figure the contrast function leads "7 •.....•'""•,-•'.':'i.-":'"-":'.:".-:" '.".'"".:"7:"'" • '•.' '- .: -.._ '.. =•. 7.•:'&' • : ." :"' . .... g ::. 2 ß ..:. ß : '. •. ... :• .:. •' ...-..:• .... ... .. . . .... . . . . . . .-" '• .7 -' . -:..:•. • ": .& '"!:.. Figure 8.•Surface of human incisor treated with coarse abra- sive (reflected light phase contrast: $00X).

QUANTITATIVE MICROSCOPY 495 to a sensitivity of detection of optic path differences of around 15 A. for reflected light phase contrast. However, for practical purposes, a figure of approximately 25 A. should be considered the limit. Incident phase contrast thus offers a possibility to observe the very beginning of corrosion (Fig. 7) in containers or on container material, the uniformity of sprayed-on films and protective coatings, or the effect of coarse abrasives on tooth surfaces (Fig. 8). Phase contrast techniques are primarily methods of detection. Minute differences of optical paths caused by relief or change in refractive index in the specimen are made visible. Sudden changes of optic path are particu- larly accentuated. However, halo formation prevents quantitative evalua- tion of the intensity distribution in a phase contrast image. Exceptions to this rule are techniques which use the disappearance of the halo to de- tect a match of refractive indices, e.g., between specimen and medium (10-12). INTERFERENCEMICROSCOPY Interference methods are especially valuable tiecause they yield quanti- tative results. The image in an interference microscope shows the speci- men and a superiraposed interference fringe system. In any interference microscope the light is separated into at least two coherent beams. One of these is called the measuring beam, the other the reference beam. How this separation is achieved depends on the specific design of the instru- ment. The light in the measuring beam is affected by the specimen the reference beam is given an optical path that will leave it either com- pletely unaffected by the specimen or which keeps the specimen influence at a minimum. Behind the object plane the two coherent beams are re- united and brought to interference. Even without a specimen in the microscope, one observes a system of interference fringes: in monochro- matic light, a dark fringe where conditions prevail for destructive inter- ference between measuring and reference beam, and a bright fringe where conditions exist for constructive interference between these two beams. Since these conditions occur alternately and periodically for optic path differences between the two beams of an uneven number of half-wave- lengths (destructive interference, 180 ø phase shift), and of an even number of half-wavelengths (constructive interference, 0 ø or 360 ø phase shift), the distance from one dark fringe to the next represents a path difference of exactly one wavelength. A specimen which introduces additional path differences for these rays in the measuring beam which have passed through it, will, in effect, change the location where conditions for destructive and constructive interference