34 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



Experiment VL — 



Card diaphragms, i / and i g are now to be substituted for the 

 slit. In these the openings are circular and central, and are so 

 apportioned to the coarse and fine groups of crossed lines, that the 

 larger circle used with the finer grating, and the smaller circle with 

 the coarser grating exclude all spectra but those lying next situate 

 to the central direct image. The spectra of the two groups of 

 crossed lines are seen as in pi. III., figs, 14 (for the lines inclined at 

 60*^) and 15 (for the lines crossing at right angles.) When the 

 diaphragms are used (i y for fig. jo, and t/for fig 11) it will be 

 seen that the pattern of the rectangular group has become meshed 

 like Pleiirosigma Balticum, and that of the inclined group has 

 become a hexagon meshed pattern exactly like pi. III., fig. 16. 

 {Pleiirosigma angiilatum). This experiment strikingly illustrates 

 the fact that the figure of Pleuros : angulatum is not necessarily the 

 transcript of an actual material structure, but is adequately 

 explained by the circumstance that objectives of all (even the 

 highest) angular apertures admit only the first series of diffraction 

 pencils, i.e., those next situate to the direct pencil, just as the 

 diaphragm cards do in the present case of groups of ruled lines, 

 (pi. III., figs. 14 and 15). 



Experiment VII. — 



The same object is viewed with the same illumination but with 

 diaphragm i 7^, and there will be seen the same hexagonal fields on 

 the fine group of inclined lines. But for this effect a particular 

 position of the three holes in the card diaphragm (i A) is necessary, 

 namely, a position in which three diffraction pencils fall with direct 

 incidence on the openings. This experiment shews that the 

 simultaneous entrance of three separate pencils is needed to produce 

 the hexagonal fields. 



Experiments with PI. angulatxim. — 

 The diffraction spectrum of this object, when examined with 

 central illumination and an immersion objective, has the form given 



