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.
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_.
In the same way, if we stop off all but the outer spectra, as in Fig. 30, the lines are apparently again doubled, as seen in Fig. 30_a_.
A case of apparent creation of structure, similar in principle to the foregoing, though more striking, is afforded by a network of squares, as in Fig. 31, having sides _parallel_ to this page, which gives the spectra shown in Fig. 31_a_, consisting of vertical rows for the horizontal lines and horizontal rows for the vertical ones. But it is readily seen that two diagonal rows of spectra exist at right angles to the diagonals of the squares, just as would arise from sets of lines in the direction of the diagonals, so that if the theory holds good we ought to find, on obstructing all the other spectra and allowing only the diagonal ones to pass to the eye-piece, that the vertical and horizontal lines have disappeared and are replaced by two new sets of lines at _right angles to the diagonals_.
On inserting the diaphragm, Fig. 32, and replacing the eye-piece, we find in the place of the old network the one shown in Fig. 32_a_, the squares being, however, smaller in the proportion of 1 : √2, as they should be in accordance with the theory propounded.
An object such as _Pleurosigma angulatum_, which gives six diffraction spectra arranged as in Fig. 33, should, according to this theory, show markings in a hexagonal arrangement. For there will be one set of lines at right angles to _b_, _a_, _e_, another set at right angles to _c_, _a_, _f_, and a third at right angles to _g_, _a_, _d_. These three sets of lines will obviously produce the appearance shown in Fig. 33_a_.