ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



351 



confine ourselves here to a few only out of the numerous cases 

 in whicli the special influence of the diflEraction spectra may be 

 demonstrated. 



The first experiment shows that with, for instance, the central 

 beam, or any one of the spectral beams alone, only the contour of the 

 object is seen, the addition of at least one diffraction spectrum being 

 essential to the visibility of the structure. 



Fig. 93 shows the appearance presented by an object composed of 

 wide and narrov/ lines ruled on glass, when viewed in the ordinary 



Fig. 93. 



Fig. 94. 



way with the eye-piece in place, and Fig. 94 the appearance pre- 

 sented at the back of the objective, when the eye-piece is removed, 

 the spectra being ranged on either side of the central (white) image, 

 and at right angles to the direction of the lines ; in accordance with 

 theory, they are farther apart for the fine lines than for the wide 

 ones. 



If now, by a diaphragm at the back of the objective, like Fig. 95, 

 we cover up all the diffraction spectra, allowing only the direct rays 

 to reach the image, the object will appear to be wholly deprived of 



Fig. 95. 



Fig. 96. 



fine details, only the outline remaining and every delineation of 

 minute structure disappearing just as if the Microscope had suddenly 

 lost its optical power, see Fig. 96. 



This illustrates a case of the obliteration of structure by obstruct- 

 ing the passage of the diffraction spectra to the eye-piece. 



The second experiment shows how the appearance of fine structure 

 may be created by manipulating the spectra. 



If a diaphragm such as that shown in Fig. 97 is placed at the back 



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