HISTORICAL BACKGROUND 3 



details of many biological and industrial specimens are characterized by 

 differences in refractive index rather than by differences in light absorp- 

 tion. Under an ordinary microscope such details are invisible, unless 

 the aperture of the condenser or objective is made so small that the 

 resolving power suffers a serious deterioration with resultant loss in 

 the observer's ability to interpret what he sees. 



Light can be considered a form of wave motion consisting of sinusoidal 

 waves. When a light w^ave traverses a medium of different optical path 

 the phase of the light wave is altered. This alteration may be visualized 

 as simply a displacement of the wave in its direction of propagation. 

 If a microscopic specimen contains details that differ from each other in 

 optical path, the phase of that portion of the illuminating wave front 

 passing through the detail is changed. If a microscope can delineate 

 change of phase as a change in brightness or color, the eye, photographic 

 plate, or photocell will be able to detect the microscopic areas causing 

 the phase changes. The phase microscope does this, and it makes 

 available another type of microscope specifically designed to utilize a 

 particular optical property of the specimen. 



2. HISTORICAL BACKGROUND 



The formation of images in the microscope is an effect of the diffraction 

 of light. In this connection the classical experiments of Abbe are 

 y3rought to mind. In these experiments are found the first reported 

 facts on the effect of introducing phase changes on light waves within 

 the optical system of the microscope. Abbe, according to Bratuscheck 

 (1892), introduced glass wedges into the rear focal plane of the micro- 

 scope objective and thereby changed the phase relationships existing 

 between the diffraction spectra produced by a grating, having opaque 

 lines, which was used as the microscopic specimen. He found that the 

 contrast between the lines and their spaces could be reversed when he 

 introduced a phase difference of 180° between the light waves composing 

 the zero- and first-order diffraction spectra. Later Bratuscheck in- 

 troduced absorbing strips of soot at the back focal plane of the micro- 

 scope objective to weaken the brightness of the zero-order spectrum. 

 As an object specimen he used alternate clear and slightly absorbing 

 strips of soot. The strips of soot were so lightly coated that they were 

 almost transparent and, hence, difficult to see with the ordinary micro- 

 scope. By means of the above-mentioned absorbing plate in the rear 

 focal plane of the objective, the visibility of the strips was greatly en- 

 hanced. Thus, he proved that by weakening the zero order of the 

 diffraction spectra of this nearly transparent specimen the contrast in 

 the image could be increased. Not only was Bratuscheck able to in- 



