An invaluable property of quartz is its power of insulating perfectly, even in an atmosphere saturated with water.  The 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 the very steep angle at which the bands run up the spectrum.

Into the theory of the development of these bands I am unable to enter; that is a subject on which your professor of natural philosophy is best able to speak.  Perhaps I may venture to express the hope, as the experimental investigation of this subject is now rendered possible, that he may be induced to carry out a research for which he is so eminently fitted.