DIFFRACTION EXPERIMENTS 



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subject may do so most profitably by the study of the only book in 

 our language that deals exhaustively with the theory of modern 

 microscopical optics, viz. the translation of Naegeli and Schwendener's 

 ' Microscope in Theory and Practice,' translated and placed within 

 the reach of English microscopists by the joint labour of Mr. Frank 

 Crisp and Mr. John Mayall, jun. The experimental proof of the 

 diffraction theory of microscopic vision lies within the range of our 



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FIG. 50. Diffraction grating 



FIG. 51. Diffraction image at back 

 of lens without eye-piece. 



purpose, and the following experiments will suffice to show those who 

 possess the instruments, and desire the evidence, that to the action 

 of diffraction spectra we are indebted for microscopical delineation. 



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. 50 shows the appearance presented by an object composed 

 of wide and narrow lines ruled on glass when viewed in the ordinary 

 way with the eye-piece in place, and fig. 51 the appearance presented 

 at the back of the objective w r hen the eye-piece is removed, the 



FIG. 52. 



FIG. 58. 



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. 52, 

 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 



