PROTOPLASM 7 



MICROSCOPIC VISION 



Abbe, as is well known, attempted to reduce the formation of all images by the 

 microscope to phenomena of difiraction. There is no doubt of the importance of 

 this point of view, but, under correct methods of illumination, diffraction may be 

 reduced so far that other modes of vision, refraction, and absorption, are pre- 

 ponderant. We will first consider diffraction. This phenomenon is due to the 

 wave form in which light is propagated. It may be roughly described as the 

 property of waves to bend round corners. Sound can be heard from a street at an 

 angle to that in which it is produced, and the fact is sometimes disturbing, just as 

 the corresponding phenomenon in vision through the microscope is. Another 

 instructive fact is shown by the following case : 



Suppose a deep bay, narrowed at its opening to the sea by two stone jetties 

 projecting from each side, and leaving only a small passage between them (Fig. 7). 



FIG. 7. DIAGRAM TO ILLUSTRATE DIFFRACTION OF WAVES OF~THE 

 SEA ENTERING A HARBOUR. 





Waves approaching from the open sea pass through the gap, and spread out inside 

 the harbour somewhat as represented in the diagram. An observer at A, supposing 

 that he did not look at the opening, would obtain no evidence of its width from 

 the waves arriving at his feet. . On the other hand, supposing that a close fire of 

 bullets were directed at right angles to the jetties, those that reached a cliff face 

 at A would show the width of the opening. 



In a similar manner light waves bend round the edges of objects, and diminish 

 the sharpness with which images of these objects are formed on the retina. 

 Blurred and incorrect definition of the boundaries of objects are only too frequently 

 seen in published photographs of microscopic preparations. When such prepara- 

 tions have a regular pattern, such as the shells of diatoms, a number of totally 

 distinct images may be formed according to the position of the objective. 



Apathy (1901, p. 514) describes the following experiment. A diatom of coarse structure, 

 such as Triceratium famis, is observed by an apochromatic objective of 16 mm. focus, and 

 ocular 8, 12, or 18. The substage iris is narrowed to 0'5 mm. in order to give a narrow cone of 

 light. It will be found that no less than fifteen distinctly different images can be seen, as 

 the objective is raised and lowered by the fine adjustment. These images are situated in 

 the course of a movement of about 250 p, whereas the total thickness of the diatom is only 4 n, 

 so that they cannot be due to differences of structure in the depth of the diatom itself. At the 



