84 Transactions of the Royal Microscopical Society. 
placed beneath the objective, in the middle of the field, the set 
arising from the wider portion, being eaactly half the distance 
apart of that arising from the narrower, such distances between the 
spectra, being inversely proportional to the distances between the 
lines themselves. 
On removal of the eye-piece these two rows of spectra (Fig. 2) 
are visible, one above the other, as the eye is brought to see succes- 
sively the air images at the upper end of the tube. 
It is obvious, from the figure, that as the wider grating gives 
spectra exactly half the distance apart, and therefore twice as 
numerous as those arising from the narrower, that the latter may 
be made to coincide with the former, in number and position (as 
required), by stopping out every alternate ray from the wider 
grating, beginning with the first. 
This is readily accomplished by placing a stop close to the 
back combination of the objective, so constructed that a central 
slit will admit the central ray only, whilst another slit on each 
side will admit only the second spectrum of the wider and the first 
spectrum of the narrower grating (Fig. 3). 
On replacing the eye-piece it will now be seen that the micro- 
scopic image of the narrower lines remains unaltered, but that the 
wider lines have doubled in number (Fig. 4), by an apparent pro- 
longation of the shorter lines between them, making the two images 
identical, the upper part being distinguishable only from the lower 
by somewhat less brightness, which simply arises from the smaller 
number of real lines through which the light can pass. 
Again, by stopping out all the spectra, except the fourth of 
the wider, and the second of the narrower (Fig. 9), the spectral 
aspect is again rendered similar in the two cases, and the micro- 
scopic images, though changed, will be still found to be identical, 
by the doubling of the narrower and quadrupling of the coarser 
lines (Fig. 10); but to see this distinctly, with so low a power as 
Zeiss’ a, a, the fifth, or E, eye-piece is required. 
I should state that, although in this experiment a Zeiss a, a, 
with a third eye-piece, has been used, any other object-glass of 
about 1 or 2 inch focus would do as well with a suitable stop; 
the stop used with the objective mentioned in this experiment has 
three slits, each 3, of an inch wide, with the same distance between 
them. 
2nd Experiment.—In this experiment the same grating is used 
as before, with a diaphragm having a single central slit, so ad- 
justed (parallel to the lines of the object) that one spectrum only 
will be admitted on each side, from the coarser grating, and none 
whatever from the finer (Fig. 5); the object of the experiment 
being to show that unless one spectrum at least is admitted there 
is no power to resolve the lines. 
