Fig. 29.
Fig. 29a.
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.
Fig. 30.
Fig. 30a.
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.