S. Kensington,

3rd Nov., 1890.

Dear Sir George,

In the summer you asked me to let you have some quartz fibre. I now leave a box containing one long one wound 18 times up and down the frame. The front and back of the box are of glass so that the fibre may be examined without risk, and black paper is placed at one end in case a dark background may be required.

The glass may be taken out after first removing a screw at the end, and then the frame can be removed.

The fibre is good, in that under the prism test it shows both perfectly uniform pieces and places where the variation in diameter gives rise to small [i.e. short but pronounced and jagged] bends in the dark bands of the spectrum.

To see these spectra to perfection the following is the best way to hold the box and a prism: first see the colours with the naked eye [with light from a slit, the box intervening] and then hold a low-angle prism in front of the eye, not in the natural position of minimum deviation, but inclined so that incident light falls at about the polarising angle. It is only thus that the dark bands appear really dark.

On then moving the box along its own length so as to make successive parts of the fibres occupy the same position, you will see some of the fibres giving a constant spectrum which is the test of uniformity, while some show the irregularities described.