several spectra in view at the same time, but the dispersion
can be varied continuously by sloping the grating. The slits may be
cut out of tin-plate, and half covered by mica or "microscopic glass,"
held in position by a little cement.

If a telescope be employed there is a distinction to be observed,
according as the half-covered aperture is between the eye and the
ocular, or in front of the object-glass. In the former case the
function of the telescope is simply to increase the dispersion, and
the formation of the bands is of course independent of the particular
manner in which the dispersion arises. If, however, the half-covered
aperture be in front of the object-glass, the phenomenon is magnified
as a whole, and the desirable relation between the (unmagnified)
dispersion and the aperture is the same as without the telescope.
There appears to be no further advantage in the use of a telescope
than the increased facility of accommodation, and for this of course a
very low power suffices.

The original investigation of Stokes, here briefly sketched, extends
also to the case where the streams are of unequal width h, k, and are
separated by an interval 2g. In the case of unequal width the bands
cannot be black; but if h = k, the finiteness of 2g does not preclude
the formation of black bands.

The theory of Talbot's bands with a half-covered _circular_ aperture
has been considered by H. Struve (_St Peters. Trans._, 1883, 31, No.
1).

The subject of "Talbot's bands" has been treated in a very instructive
manner by A. Schuster (_Phil. Mag._, 1904), whose point of view offers
the great advantage of affording an instantaneous explanation of the
peculiarity noticed by Brewster. A plane _pulse_, i.e. a disturbance
limited to an infinitely thin slice of the medium, is supposed to fall
upon a parallel grating, which again may be regarded as formed of
infinitely thin wires, or infinitely narrow lines traced upon glass.
The secondary pulses diverted by the ruling fall upon an object-glass
as usual, and on arrival at the focus constitute a procession equally
spaced in time, the interval between consecutive members depending
upon the obliquity. If a retarding plate be now inserted so as to
operate upon the pulses which come from one side of the grating, while
leaving the remainder unaffected, we have to consider what happens at
the focal point chosen. A f