1898.] MICROSCOPICAL JOURNAL. 113 



magnificently shown in Mr. Nelson's photograph with a 

 wide cone. 



17. We can also, however, see the large amount of 

 truth in the Abbe theory, and its important, though not 

 aZZ-important, place in microscopic vision, especially for 

 certain classes of objects. Wherever we have a known 

 periodic structure in transparent objects, plane-wave 

 illumination and the consequent interference-lines formed 

 by the beams diffracted by that structure, have an extra- 

 ordinary effect in intensifying into black and white a 

 more or less accurate representation of the periodic 

 detail. How this occurs can be easily seen from two 

 examples, macroscopic and microscopic. 



Take first quite a coarse striation of 50 to the inch, 

 visible to the naked eye, represented by a grating of 

 platinum wire and by a piece of platinum foil corrugated 

 to the same gauge. Make the wire incandescent, and 

 (checking irradiation by a smoked glass) the striation is 

 easily seen. Make the corrugated foil incandescent 

 (these observations are supposed to be in the dark) and 

 probably the detail will be quite invisible. The eye was 

 quite competent to see structure of this fineness by the 

 Airy self-luminous method, if the detail was in contrast; 

 but there is now no contrast, and the detail is more or 

 less invisible. Then let the corrugated foil be cold and 

 illuminated by extraneous light, and the detail is seen 

 again. There is both shadow to assist the contrasts, and 

 also there are phase-relations between the tops and bot- 

 toms of the striations which come into play. 



Let us further imagine a perfectly transparent struc- 

 ture with uniform periodic detail, but the elements of 

 that detail differing in thickness only ; and let it be 

 mounted in a medium of nearly the same refractive 

 index. A diatom in balsam nearly represents such a 

 case. It is quite evident that by heterogeneous illumi- 

 nation at all approaching the self-luminous character, it 



