Fig. 27a.
The influence of these diffraction spectra may be demonstrated by some very striking experiments, which show that they are not by any means accidental phenomena, but are directly connected with the image which is seen by the eye.
The first experiment shows that with 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. 28.
Fig. 28a.
When by a diaphragm placed at the back of the objective, as in [Fig. 28], we cover up all the diffraction spectra of [Fig. 27]a, and allow only the central rays to reach the image, the object will appear to be wholly deprived of fine details, the outline alone will remain, and every delineation of minute structure will disappear, just as if the microscope had suddenly lost its optical power, as in [Fig. 28]a.
This experiment illustrates a case of the obliteration of structure by obstructing the passage of the diffraction spectra to the eye-piece. The next experiment shows how the appearance of fine structure may be created by manipulating the spectra.