When a diaphragm opening is interposed between the mirror, and a plate of ruled lines placed upon the stage such as Fig. 27, the appearance shown in Fig. 27_a_, will be observed at the back of the objective on removing the eye-piece and looking down the tube of the microscope. The centre circles are the images of the diaphragm opening produced by the direct rays, while those on the other side (always at right angles to the direction of the lines) are the diffraction images produced by the rays which are bent off from the incident pencil. In homogeneous light the central and lateral images agree in size and form, but in white light the diffraction images are radially drawn out, with the outer edges red and the inner blue (the reverse of the ordinary spectrum), forming, in fact, regular spectra the distance separating each of which varies inversely as the closeness of the lines, being for instance with the same objective twice as far apart when the lines are twice as close. [Illustration: Fig. 27.] [Illustration: Fig. 27_a_.] 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. [Illustration: Fig. 28.] [Illustration: Fig. 28_a_.] 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. [Illustration: Fig. 29.] [Illustration: Fig. 29_a_.] When a diaphragm such as that shown in Fig. 29 is placed at the back of the objective, so as to cut off each alternate one of the upper row of spectra in Fig. 27_a_, that row will obviously become identical with the lower one, and if the theory holds good, we should find the image of the upper lines identical with that of the lower. On replacing the eye-piece, we see that it is so, the upper set of lines are doubled in number, a new line appearing in the centre of the space between each of the old (upper) ones, and upper and lower set having become to all appearance identical, as seen in Fig. 29_a_. [Illustration: Fig. 30.] [Illustration: Fig. 30_a_.]