e gold leaves now
diverging were charged some time before the lecture, and hardly show
any change, yet the insulator is a rod of quartz only three-quarters
of an inch long, and the air is kept moist by a dish of water. The
quartz may even be dipped in the water and replaced with the water
upon it without any difference in the insulation being observed.

Not only can fibers be made of extreme fineness, but they are
wonderfully uniform in diameter. So uniform are they that they
perfectly stand an optical test so severe that irregularities
invisible in any microscope would immediately be made apparent. Every
one must have noticed when the sun is shining upon a border of flowers
and shrubs how the lines which spiders use as railways to travel from
place to place glisten with brilliant colors. These colors are only
produced when the fibers are sufficiently fine. If you take one of
these webs and examine it in the sunlight, you will find that the
colors are variegated, and the effect, consequently, is one of great
beauty.

A quartz fiber of about the same size shows colors in the same way,
but the tint is perfectly uniform on the fiber. If the color of the
fiber is examined with a prism, the spectrum is found to consist of
alternate bright and dark bands. Upon the screen are photographs
taken by Mr. Briscoe, a student in the laboratory at South Kensington,
of the spectra of some of these fibers at different angles of
incidence. It will be seen that coarse fibers have more bands than
fine, and that the number increases with the angle of incidence of the
light. There are peculiarities in the march of the bands as the angle
increases which I cannot describe now. I may only say that they appear
to move not uniformly, but in waves, presenting very much the
appearance of a caterpillar walking.

So uniform are the quartz fibers that the spectrum from end to end
consists of parallel bands. Occasionally a fiber is found which
presents a slight irregularity here and there. A spider line is so
irregular that these bands are hardly observable; but, as the
photograph on the screen shows, it is possible to trace them running
up and down the spectrum when you know what to look for.

To show that these longitudinal bands are due to the irregularities, I
have drawn a taper piece of quartz by hand, in which the two edges
make with one another an almost imperceptible angle, and the spectrum
of this shows the gradual change of diameter by t